Exactly, and earlier Bf 109s did have a variable speed airscrew. The control, like a rocker switch, was positioned on the throttle lever and the pitch indicator was the dial at the lower right of the instrument panel, sensibly next to the engine tachometer.
Whilst a manually controlled variable pitch unit is much superior to a fixed or two pitch system it did need training to be used correctly. There are accounts from the latter stages of the BoB which show that some of the Luftwaffe's inexperienced replacement pilots struggled to operate the system even when flying in formation, never mind under the stress of combat. In this sense a constant speed unit gives an advantage.
And even so, a basic 2-pitch system is still better than fixed pitch.
Well, the Rotol was constant speed and the DH 2 position props were converted to variable between the limits if not full constant speed (to avoid mind reading I mean I am not sure, the conversion may have been to full constant speed operation). The thing is that fitting such propellers in the summer of 1940 was 3-4 years late. Fitting 2 pitch propellers in 1939 was 5-6 years late. The British should have been in the leading ranks of the change over if they couldn't be the first, not being dragged kicking and screaming into modern propellers
Yes, the likes of Kestrel and Mercury fighters should have been adopting DH 2-pitch props in the early 1930s alongside their airliners, racers, and mail planes. (possibly 2-bladed units similar to those later used with the Gipsy Twelve ... or the Jumo 210 powered 109s)
The British had experimented with a variety of variable pitch designs back in the 20s and found a number of drawbacks/problems. This colored their thinking in the 1930s despite an almost universal adoption of the variable pitch and constant speed propellers by just about every other major country.
That's very slightly reminiscent of the RAF .50 cal gun trials where .303 was seen as more efficient and 20 mm as the more realistic follow-on and no interest was given in replacing the .50 vickers with a browning derivative firing the same cartidge. (granted, had they hedged their bets for the hispano with the Oerlikon FFF -or possibly FFL- rather than the FFS with bulk, weight and recoil closer to the Hispano, that may have been a non-issue -I find it very unlikely that 4x FFF cannons wouldn't have been satisfactory in the Hurricane Mk.I)
The Hispano might be a good choice, the FW 187 might not need much, if any beefing up as it was designed to use DB engines in the first place and only used the Jumos because of DB shortages.
The Gnome-Rhones are interesting but they will NEVER put out the power you see listed in Wiki. Please remember the 14N is a two bearing engine. the 14R got the center bearing and gained hundreds of pounds of weight.
See Russian M-88 engine for practical power with better than 87 octane fuel and a two speed supercharger.
The Hispano engine is definitely an interesting option and perhaps more attractive than the Jumo 211, at least as a fighter. (a little less so as a fighter-bomber)
The 14N wouldn't be that much more attractive than the Bramo 323 and unlike the Bramo (but like the Hispano) wouldn't have been a pre-war option at all. Granted, a 12Y powered 187 could have entered production prior to the Battle of Britain, perhaps even early enough to serve there (certainly in time to be useful in Norway).
The only reason I seriously mentioned the Bramo engine was as an alternative to the Jumo 210, nothing else. The resistance to ground fire would be an added bonus, but not a deciding factor.
the XP-42 suffered from both vibration and cooling issues. Something else is that the Army wanted around 500 fighters (it ordered 524 P-40s initially) and Seversky had built under 100 planes in the companies existence and Bell had built under 20. That left Lockheed and Curtiss as established mass manufacturers. Since the P-36, XP-37, H75R, XP-42 and XP_40 all used essentially the same wing, tail and landing gear and Curtiss had built 210 P-36s and had orders for 200 Hawk 75s for France on the books in the spring of 1939 while Lockheed, despite building several hundred twin engine Airliners and Hudsons had no production set up for the P-38. Selection of the P-40 seems to have been a no-brainer, delivers to squadrons could start months if not a year earlier than most of the other contenders.
The early cowling tests on the XP-42 were pretty worthless, it's only the later refinements to using a short, tight cowling with fan and large spinner (not unlike that of the Fw 190, XP-47J or Tempest II) that it was actually worthwhile.
And while the P-40 was certainly compelling and worth production, my point was that continuing development of radial-engined counterparts would still have made sense. By the time of the P-40B/C, the Wildcat had already entered service, and by the time the improved cowling of the XP-42 was tested the initial shortages of 2-stage engines would have been well resolved. (not only that, but R-2000s would have been around) As far as I'm aware, the late XP-42 cowling was never mated with a 2-stage R-1830 for testing. (granted, for ground attack, the better low alt performance of the R-2000 would be more interesting, but by that time other aircraft were probably much more useful as fighters or fighter-bombers)
As to the .50 cowl guns, yes for ground attack replacing those with a pair of .30 guns probably would have been more effective, though in terms of operational use, .50s seemed to be preferred in spite of the disadvantages. (or at least the Finnish Air Force seemed to very much like their cowl mounted .50 brownings -replacing the .30s on both their Hawks and Buffalos with them, and preferring all cowl mounted .50 brownings for their Myrsky)
Another BTW, the figures in wiki for the XF5F are way off if that is what you are going by, those are projected figures which were never reached and the plane never flew with armament installed.
Indeed, I rather doubt the Bramo powered 187 would have managed much beyond 340 MPH and a significant decrease in range, but improved max takeoff weight, somewhat improved climb and slightly superior speed to the Hurricane and Bf 110. (without water injection, the 2-speed Bramos managed 100-175 PS less than the DB-601A depending on altitude) As a fighter, I'm not sure the losses in range and -likely- roll rate plus reduced cockpit visibility would be worthwhile, especially with weight savings in a single-seat configuration. Plus the single-seat prototypes were never tested with the slightly better performing 210G engine. (the 2-seater models also resorted to simpler fixed radiators rather than the more streamlined retractable units)
And I do realize the shopping for missions issue is akin fot what Focke Wulf did, and it may have been just as futile if RLM politics didn't change any. (alternate attempts to interest Udet notwithstanding)
That said, there is one area regarding both the heavy fighter and fighter-bomber concept that hasn't come up yet, and that's the issue of the 'fighter bomber' and 'heavy fighter' concepts from the mid 1930s or even early 1940s centering much more on taking twin engine light bombers and giving them anti-bomber armaments, or similarly creating aircraft intended to be both light bombers AND bomber destroyers AND ground attack aircraft.
This proved unworkable with the Kampfzerstorer concept and Messerschmitt managed to sway the RLM to shift towards the Zerstorer concept (though oddly without even considering provisions to mount external bombs, apparently). The Fw 187 wasn't designed with that new requirement in mind, but nevertheless was an even further departure from the bulky 'bomber as heavy fighter' concept and did away with the need for defensive armament. (albeit in true practical terms, with the inability to re-load the cannon drums and with bomber interception in mind, making modifications to allow a full 4 MG-FF cannons -perhaps omitting LMGs entirely- would have been more compelling)
And even then, the Ju 88C ended up pressed into more or less the same role the mid 1930s Kampfzerstorer concept failed in, admittedly more akin to the likes of the early A-20 at that point, but underpowered. (though I still think the Ju 88 might have fared better in a more Mosquito-like configuration -if still not quite that capable, especially with BMW 801s)
and 'adjust' the pitch that way as opposed to fix pitch props even if metal. 
