I don't think leaning on the backstay would do that. Mast boot is usually held on with big SS hose clamps one around the mast and the other around the casting that forms the partners. I usually put a bead of sealant, not adhesive, on the joint at the mast before I put on and tighten the top clamp but I don't do anything to the bottom one other than tighten the clamp. The boot itself could have a pinhole some of them are cheap and only last a year or two.

While you have your chainplates apart you should check to see if the core has been ground out and epoxied in there and if not you should do it. It could save you a huge pile of money down the road from a rotten core. It is not always apparent with this boats that you have a leak, especially with the cap shroud chainplates and the water will do a lot of damage to the core and bulkheads. There is a new style chainplate deck cover for the cap shroud as well and it is worth looking into.

The backstay takes a huge load, and your puny body (no offense...or my 192 lb puny body) leaning against it is a total non-event.

Re the leaky chainplates, search on this forum for the thread posts by one of our Forum contributors "Mainesail". There you'll find a link to his website where he has great step-by-step through deck sealing How To's. read through teh discussions in this Forum first, then go to his website for the confidence to do this job correctly with the proper sealant.