Nobody's answered yet, so I'll try.
If the pattern doesn't say how many stitches to catch I would just eyeball it. Knitting and crochet gauges are completely different so it's really hard to give a specific number. Hopefully the pattern says what size hook to use for the edging; if not, you'd have to "play" with that too until you get an edging that looks good and doesn't make the bag either stretch out or draw in there.
You'll probably have to try it (meaning "rip it out"
) a few times before you get it right. But I think that once you figure it out you'll get a rhythm going - like "1 sc every x stitches" - and it'll get easier.
Crochet does make a thicker fabric than knitting and that had me wondering about using different yarn for the edging. But fabric usually gets thicker when it's felted so maybe that will make up for the difference.
As for the button loop - I do think a chain would be a good choice. If the pattern tells how many sc's to allow for the loop you'll probably have to do more chains than that because sc's are usually fatter. If it doesn't, you could make an educated guess based on the size of the button you'll be using for the closure. Just keep in mind that after the bag is felted everything will be smaller - including the loop - so you'll have to make the loop bigger than you normally would.
It's hard for me to more specific because I have no experience with felting. But I hope that helps you decide what to do. Good luck.