1-must wear straight shoe lasts for overpronation & flat feet? 2-can sneakers have mesh and leather tops?

?? Not so much a straight last as much as a last of some kind / cheaper shoes may look ok but if you can bend the shoe easily at the heel-forefoot junction then this is not a good shoe for you a good shoe with a last coupled with a good otc or custom made orthotic is the ticket as long as the construction is of reasonable quality what the upper is made from is not too much of an issue.
Actually. The material does not matter and a straight last wont necessarily matter. Just try an orthotic, if made right this should work.
Custom insoles. The best correction for over pronation would be the use of custom orthoses (insoles) for your feet. There are shoes that help correct this, by using what is called a medial fill that is built into the shoe. Custom insoles are made from a prescription and require a cast impression. This can help alleviate alignment issues from the feet up to the spine.