Cutting down the fins is probably the easiest & fastest change to implement.
Yes but it requires several things. Technically it was easy to trim the fins, The hard part was convincing the RAF that they actually needed 500lb bombs

In 1940 they made about 4 times the number of 250lbs GP bombs than they did 500lb GP bombs. It 1941 it was roughly equal and production vs actual use may have varied considerably in 1942, The British were also shifting to MC case bombs and 1000lb bombs and incendiaries in 1942.
Better start making these "Mosquitoes minus" ASAP.
German fighters of 1939-40 are also much, much slower than what was case in 1942, talk 50 mph deficit (greater for the Bf 110s). Bf 109E also has no endurance chasing the actually fast bombers.
Battle with Merlin X will still be well under 300 mph.
The Mosquito lite will be more effective than the older light bombers, just don't expect quite the same performance (either speed or load) that the Merlin 21 powered versions had.
Early Mosquitos were used for recon and for nightfighters. The Night fighter role could be done by the Mosquito lite, but here there are two differences, Most of the Luftwaffe flies of to Russia in May/June of 1941 leaving a lack of targets. 2nd it takes until April/May of 1941 for the radar and ground controllers to come together. A 340mph Mosquito light is not going to give much better results than a Beaufighter because the radar and technique needed more work.
A slow bomber operating without escort is a no-no. Night operations are something else.
Service ceiling at max weight of Halifax I was at 18000 ft. Compares well with with Whitley V (same engine) at 17600, that of Whitley VIII at 16000 ft, or Wellington I at 15000 ft. Yes, Wellington III is better (19500), but it carries 3 times less the bombs' weight.
You are correct but it shows that unless RAF thinking changed somewhat higher performance aircraft could/would be miss-used.
Planes will burn off some weight and not be operating at max gross over targets, but again, lower the ceiling of the Lancaster by 3,000ft and the speed by 20mph or so. It is better but not a big game changer.
On the Battle and Blenheim I, there was almost nothing to remove - there was no protection, a pair of guns was present, meager bomb load, not much of fuel. What was there were the too big and too draggy airframes - these can't be dealt with nip & tuck. Engine power vs. what was required from it was laughable.
Later Blenheims gained protection, fuel and firepower, performance went from bad to appalling.
I was actually referring to a Mosquito lite or Lancaster lite. If you need to take-off several thousand pounds lighter due to the less powerful engines what do you leave out of the lower powered Mosquitos and Lancasters that some people want to build?
Especially if you try to backdate them to use Melrin II-V engines out of Battles or Defiants.
They only built about 350 Defiant IIs with Merlin XX engines so there isn't a big pool of engines to use for this scheme.
RR built about 4900 Merlin X engines but these were used to power about 400 Wellington IIs and around 1700 (?) Whitleys so 4200 plus the couple score of Halifax's plus spare engines.
No idea how much trouble to would have been to convert single speed Merlin production to two speed in 1939 and early 1940. However Derby built 312 X engines and Crewe built 4,589 X engines. Ford and Glasgow don't seem to show up until the Merlin XX?
The RAF needed to cancel the Defiant in early 1939, before things got very far. But that means admitting the turret fighter was a mistake. However you do have the Hurricane and Spitfire to take up the slack

Canceling the Battle is a lot harder. You need to start making something, anything,
anything at all, really ! to replace the Hawker Harts/Hinds in 1937-38.
Hundreds of Harts/Hinds and cousins were converted to dual control trainers and were still flying in 1939-40 (and later) after being replaced by Battles.
People who want to cancel the Battle production need to have plan for replacing the Hawker biplane bomber fleet.
Now the Battles and Blenheims could have been used more effectively in several rolls than they were, and improved with somewhat minor tweaks (some of which should have been figured out in 1939. Armor trials for protection were being carried out in ranges in 1938). But that required some elements of the RAF change their thinking and that was harder than changing aircraft.
A slow bomber operating without escort is a no-no.
repeating this. The RAF believed that slow bombers could be used for ground attack/interdiction with no escort or the aid of a "fighter sweep" in the general area of and time of bomber attack. Until they got that idea out of their heads close support was going to be a disaster, which can then be used to justify the idea that close support was a waste of resources and the RAF should go back to bombing enemy cities.
Take a Fairey Battle and
1, take out the existing fuel tanks and fit smaller ones with some sort of protection ( horsehide and glue, 4mm sheet steel, something) . There is no reason for 1000 miles of range for a tactical bomber in Europe.
2. Fit 4mm-6mm armor for the rear gunner and pilot. Leave one man on the ground.
3. fit 4mm bathtub to radiator
Some sort of protection from ground fire and or 7.9mm machine gunss
4. Fit two guns in the back or fit single Browning gun with large ammo belt.
5. fit a 2nd forward firing machine gun.
Now for extra difficulty,
6. fit a low altitude supercharger (like a Merlin VIII)
7. convert two speed prop to constant speed.
8, Provide fighter escort
9. Provide fighter escort
10. provide fighter escort
It won't save France but cutting losses by even 50% would have been huge.