A couple of comments:
1. The Heinkel He 177 was hamstrung by the fact there wasn't enough money spent on fixing the engine overheating problems with the DB 610 engines. In fact, they actually modified one He 177 in early 1944 based on the analysis of known engine issues, and that modified He 177 actually functioned perfectly. Had the fixed that problem in 1942, the He 177 could have gone on to a fairly decent combat career.
A heavy bomber with 4 nacelles for 4 engines makes far more sense than what He 177 used.
BTW - DB 610 performed as advertised, it was DB 606 that was prone to spontaneous fire.
2. The Boulton-Paul Defiant was really hamstrung by the fact the plane was limited by the early Marks of the Merlin engine. Had the Defiant been upgraded to use the Merlin 45 or 61 engine, the plane would probably been a lot faster and with a redesigned turret designed to handle more powerful guns, could have been a potent anti-bomber interceptor.
Defiant, Hurricane and Spitfire in 1939-mid 1940 shared the same engine - Merlin III, difference in every-day service being that Defiant was a death trap when enemy fighters are around, unlike the other two. Due to the need to carry the turret, Defiant was wider (= draggier) than Spitfire, and heavier than either of other two. Add the drag and weight of turret, and speed and rate of climb are in problems. By 1941 (= Merlin 45 available), we have Spitfire doing 370+ mph with two cannons, and Hurricane IIC was making 320 mph with 4 cannons. Turret that can handle bigger guns = another money sink. Defiant simply does not offer anything over the cannon-armed Sptfire and Hurricane, it will be more expensive, it will require twice the crew, and it will be bad against fighters (German bombers are already fair game for any Hurricane, let alone Spitfire).
RAF has (an unlikely) surplus of Merlin 61s in 1942-43? Install them on actually useful aircraft besides Spitfire, like Mosquito, Lancaster, or re-engine the Mustangs.