"It was a bomber destroyer and in 1935/36 when the idea was produced, no one thought that Germany would invade France and be able to use single seat fighters to escort bombers into British airspace."
This is of course total nonsence.
It was predicted that France will be attacked by Germany and when Defiant will be used it could probably be attacked by enemy fighters. Defiant supporters predicted that it will be successful.
I dont know if anybody predicted that Defiant could be mixed with long range fighters like Bf 110C, but this was also possible at the end the 1930s. We need to know original documents to make any final statements towards Defiant expectations in combat.
While documents would be very helpful we also have to remember that the aerial landscape was changing at a very rapid pace. And British development was a bit on the slow side. From initial requirement to combat use in 1940 took about 5 years for the Defiant.
When the prototype Defiant first flew less than a dozen Bf 109s had been fitted with DB engines. Bf 110 production was at a creeping pace with Jumo 210 engines while they waited for DB 600 series engines in late 1938. The British were overtaken by events. Slow development/testing combined with orders being placed
before testing was completed meant large amounts of materials and tooling was already in the production pipeline.
From wiki so subject to correction. " On 28 April 1937, an initial production order for 87 aircraft was received by Boulton Paul for the P.82; as this was prior to the first flight of the prototype, the aircraft had effectively been ordered 'off the drawing board'
"By January 1940, over half of the original production batch had been completed.
[9] Beyond the initial production order in April 1937, follow-on orders had been issued for the type; in February 1938, an additional 202 Defiant Mk I aircraft were ordered; three months later, another 161 aircraft were ordered.
[9] In December 1939, yet another 150 aircraft were ordered, raising the overall total to 513. In 1940, this rose to 563 Defiant Mk Is on order, while a further 280 were ordered under a rearranged manufacturing plan issued in mid-1940"
Basically you had 450 Defiants on order before the First production example flew.
Even in 1938 the bulk of the 109s produced used Jumo 210 engines (and 235-270 ? liter fuel tanks) and were armed with four or fewer 7.9mm MGs.
People had little doubt the Germans would attack France, the doubt was how far they could advance and how close they could push the airfields used to attack England.