#1 - I've never reached a limit and haven't had any complaints about a limit
#2 - you can't do this in Proof, you have to make a comment for each change you want to indicate. Instead I would recommend you make a single comment for the designer for them do the search and replace in the native program they are using ie InDesign.
For #2, our proofreaders use the Search Text In Document tool (not search text in comment tool) and then you can use the markup tool to mark each instance of it and it is all connected to one comment box. My proofreaders love it, but my designers hate it, so that is something your people will need to work out. 😉
Hey Anthony - I have never noticed a Search Text option - is this visible when you open a Proof or do you need to be in the Document landing area?