Since filemaker announced it's deprecating runtimes in the future, you're going to find reluctance for supporting it. I'd highly recommend you not invest too much time in it, and possibly look at SBA licensing for your product instead.
SBA in the short-term would be too much expense and hassle. My potential users aren't going to part with much money per solution. Since they also mostly PC-based and need to transfer records FM Go isn't really useful either. Dropping RTs does seem unfortunate for small-time developers.
3. Charts: Any free or cheap plugins recommended for Runtime?
That depends on what you call cheap
xmCHART | X2max Software have a Runtime Licence available for $590, which also includes a single user licence for you to develop / test with.
Aside from that, you can utilise the web viewer and roll your own or use something like: Charts for FileMaker Pro