Ghosth, I see were your going with the reload timing but I think it might be a little more that 3 or 5 seconds.
1. The plane is moving around, turbulance, up and down, makes it hard for the reloader to keep his balance
2. Open the breach, remove spent shell casing (hot), discard, grab new shell, load, close the breach.
3. I dont think the smoke in the cockpit would be that big of an issue, moving at 180 to 210 mph with windows down would probably blow most of the smoke out, plus you wouldnt be refiring often enough.
for instance, I line up on a target, say a tank, probably the effective range to hit something like a tank would be 200 yards (taking in consideration trying to aim from an unstable platform it would be difficult, plus a tank is a relative small target when your moving at 180mph) Once you fire and allow for the reload time, you've overflown your target and you either hit it or you didnt. if you missed it'll probably take you a couple minutes or so to reposition your plane for another pass. If you hit it, you still might have to make another pass, 75mm is not exactly a one hit wonder on tanks.
Also, are the 75mm rounds HE or AP?
I also dont see a need to perk a B-25G or H. Why? it would not be as manuverable as the A-20 or the IL-2. I would think the destructive power of a B-25G with 21 rounds of 75mm would be close to equal of the Mossie 20mms or Me110 30mm.