A lot of these planes were ordered 1-2 years before they either flew or entered squadron service (sometimes longer) . Engines and propellers were ordered accordingly. Which makes a bit hard to swap parts around in mid production. Yes the Rotol Plant was started in 1937
As far as armor goes, A few countries (Like the Soviet Union) had begun to fit it before the war, but it was by no means common in the fall of 1939.
Neither were self sealing tanks and not all nations or companies agreed on what the best methods were. Some research had been going on between the wars (if not dating back to WWI.
However the best was the enemy of good and most countries were waiting for the ideal solution when actual war forced them to settle for 2nd or 3rd best.
from 1936 on things were changing by the year if not several times per year. Britain had orders for hundreds if not thousands of Battles and Blenheims before they ordered even a couple of hundred Wellingtons, Whitleys or Hampdens. The two light bombers had a nominal range of 1000 miles or a radius of 400 miles with about 30-60 minutes of reserve depending on speed. To escort them you would have needed a plane with similar endurance. The Hurricane had under 1/2 the endurance. The bigger twin engine bombers had even more range/radius and the Blenheim got added tankage and it's range was increased to 1400 miles in the MK IV which was equipping squadrons before the war. The range needed to escort was constantly shifting.
In 1936 they knew 100 octane was coming, what they didn't know was when (price/production capability). By late 1938 or early 1939 a lot more was known, but they still were in the dark about how much would be available. However things were getting sorted out, again with things changing almost constantly. RR had given out preliminary power figures on 100 octane fuel at the Paris Air show at the end 1938.
Given the difficulties in getting both the Hurricane and Spitfire into large scale production one does wonder at effects of trying to build special models with different fuel tanks or other accessories.
At what point was it ever considered? The early variants of the B-17 were considered good enough to operate without escorts but comically later ones weren't. The turret armed Wellington was going to shoot down defenders as quickly as they were sent up. I often wonder who tested these theories out, if indeed they did at all.
I don't know when. It is often reported that some in the AIr Ministry considered it to an impossible task. Depending on when they came to that conclusion they may have been right. However they held to that conclusion into 1941/42 at which point they were no longer correct. Technical ability had changed.
I would also note that the comedy of the B-17 was stretched out over a number of years. The Early B-17s were flying when the Germans had 109D with Jumo 210 engines, In 1940 the B-17 didn't need a lot of guns (it was thought) because it would fly at high altitude. trying to chase a B-17 or trio of them, with Bf 109E-3s was going to be pretty comical, The B-17s without gun turrets could actually out climb many fighters above 25,000ft or so. Of course then they found out that many of subsystems on the B=17 wouldn't function at that altitude. The 109s also got a lot better. By the time the later B-17s showed up they were taking off at 25,000-30,000lbs heavier than the early ones and had a lot more drag, the combination certainly made them a lot less "sprightly" shall we say
British policy was certainly scattered all over the place. A fixed gun fighter needed eight guns. Large bombers with 2 and 4 gun power turrets could shoot down large numbers of attacking fighters, four guns in power turret in a fighter was adequate to destroy large twin engine aircraft. A single pan feed machine gun was all that was needed on fast bombers for defence. Obviously not all of those positions could possibly be correct at the same time.