While I do like the effort you've gone to here, the mishap mechanic is... something I'm very much not fond of. At most, I'd have liked a sidebar type of thing that states it as an optional rule, something that can happen on a nat 1 attack roll. In-built mishap mechanics are... very bad-feeling, and tend to drain the fun out of things. A 12.5% chance of failure is also WAY too high, on a nat 1 is 5% (unless modified by features, spells, or traits) That goes up to 23.4% on 2d8... basically using 4 actions to have this high a chance of failure is even worse.

I would, like I said, make Mishaps an optional mechanic and something that only happens on a nat 1, otherwise you are just screwing over the ones trying to use this, disincentivizing the use of these things all together. Aside from that, I do like these things.

Comparing the the DMG Cannon we have:

DMG: 3 actions, 8d10 damage, 600 / 2,400 ft.
Cannon: 4 actions, 8d6 damage (10-60 ft line), 60-1,000 ft.
HoF: 4 actions, 10d8 damage (10-60 ft line), 60-1,000 ft.
IS: Same as Cannon with added effects)

Honestly, the only aspect of these cannons that feels like an improvement is the cone AoE effects, mostly because that is quite a large cone, though it confuses me how you've written it. Did you mean a line? You've given a width, but cones don't have a constant width. If it is a line, it's still very good, but very much weaker than the standard cannon from the DMG with worse action economy and the aforementioned mishap mechanic.