Ericson- Mast Compression Problem
I have an 1971 E-35. I just replaced the main bulkhead due to water leaking in the chain plates. The damage was localized at the chain plate and at the bottom of the bulkhead where it rests on the sole. The compression post was not damaged. In replacing the bulkhead I removed the mast, and compression post in order to insert the new bulkhead.
In reading your post I would suspect that the problem lies beneath the cabin sole. The compresion post sits in a form fitted slot on the sole, and is then supported underneath the sole by a large block witch may or not be in its proper place. My guess is that this block has moved or rotted away and the weight of the rig has lowered things down a bit. I do not think that overtightening the rig by itself would cause the problem you described because I think other things would break before you could get the kind of sagging you described, if compression post and below deck supporting block is in proper condition and location.
Anyway I am fairly familiar with this system you are dealing with and have access to good people to answer questions you may have... so e-mail me if you have any other questions. And I would love to find out what you find.