Just finished doing my top cover, used this site to figure out how.
Absolutely correct about the MB sealant, bought from this site. Made by locktite but could only find here, has a specific MB PART NUMBER.
If the bottom cover is loose, I am sure you would have lost seal and would need to remove, clean, and reseal surface to block. From what I've read on this site, the lower cover is no joke to remove, lots of stuff needs to come off.
I have 123000 miles and left the chain and associated hardware alone. I have other cars with chains and they run for ever over 180000 miles so far. Key is keeping clean oil and filter in car.