If you stick with a 10-second round, which is what I'd do (though in a game where a round is a round, I don't really think it matters too much whether you call it 5, 10 or 20 seconds), I would definitely heed the advice above that you should only change the rate of attack for missiles if you are okay with that greatly changing their effect in the game.
Old school D&D games have the same issue about rationalizing missile attacks, and in that world one fairly common house rule is to have each shot take up a random number of arrows, quite often 1d6. This way missile attacks are exactly like melee attacks: you take a few individual shots and somehow they add up to the overall damage roll, however you rationalize it.
However, it does has the side effect of wasting arrows much more rapidly, but I'm not sure that's a bad thing. I like games which stress resource management, and anyway, even with that rule I've rarely ever seen anyone run out of arrows in an RPG combat. If losing 1-6 arrows each attack seems excessive, you can make it 1-4 or 1-3, or do my favourite thing and roll a 'weighted' d6: say, 1-3 = 1 arrow, 4-5 = 2 arrows, 6 = 3 arrows.
Regarding the charge rule, that looks like it would work just fine. It's refreshing to see a charge rule that actually penalizes the attacker. Physically running into melee in any martial sport is usually suicide, especially if you try to go in swinging. But at the same time, this is fantasy and it's fun to leap into the fight swinging like Conan! Allowing the MM roll to mitigate the penalty and even tip it over into a bonus is a good touch.