#2
I don't know. Speakers are pretty heavy. Can't you just take on to a hardware store and get a match?

How did the screws get stripped?

Hint: when dealing with things like this unloosen the screws a just a bit evenly going from one to one until everything is loose. Putting them back in tightening a little bit going around from one to the next until all are tight. Kind of like when you change a tire on a car.

As opposed to loosening one screw at a time or tightening one screw at a time.
#4
You can run into vibration issues down the road at the least, and a speaker coming loose, dropping down and ripping the wires out at the most. So it is a good idea to go to the hardware store and gets some new ones. You can even find some new sinks to put in the baffle if you look for them. I would definitely not just let it go though. Like the guy above said... tighten them evenly, a little bit at a time. Going from one side, to the opposite side of the speaker and so on. That way it doesn't get mounted uneven. Good luck.