Restart button would give player incentive to give up early, you can work you self out from different situations if you understood mechanic, and more level is complex more different ways you can pass it (we find part of them after posting xD). So its a trade, but probably restart button would be good addition in this way: cart is speeding up 10 times while you press it, can also help skip waiting time if it isnt you 1st time on the level.
Recharge happens when you hit with last bullet in the ammo, and this is almost whole game idea, so i probably could add abilities, that let you recharge or delete all enemies, but this will ruin puzzle element or just end up mandatory to past puzzles. More ammunition can give more room for mistake, but puzzles are design around 3 ammo, so 4 can actually make them a lot more difficult.