Lubro-Moly Jectron is a fuel injector cleaner that you put in your gas tank. It's one of the few that actually work. Based on your car's history, I wouldn't be at all surprised if it made a big difference. Either way, it's not expensive and it can't hurt.
I only have one key. I'm very careful not to misplace it. Don't want to lay out hundreds for a spare.
About the understeer- check your tire pressure. 29 in front, 36 in back is the recommendation. I run 31/35 and it seems to work ok.
I used contact cement to attach the canvas when I replaced my top. A good outdoor two-sided adhesive may work too. Can be tricky getting the tension right. Wouldn't hurt to read the relevant sections from Mike Focke's top replacement DIY. You'll be hard pressed to find one better.
You're in for it now, pal. You'll be building a garage and buying a lift before you know it.
