To answer your second, yes you need to down rank your shields. On 10 man I had to down rank to (Rank 10) in order to get rapture to proc. Possibly rank 11 or 12 for 25 man.
To answer third, Start spamming bubbles 10 seconds before combat starts, you will see Infest clear your shields and that's your que to start spamming again. Watch your DBM timer and just make sure you have over half the raid shielded, if you miss one or two people before infest goes off you or your **other healer** should be able to top them off no problem.