A smaller lighter mast that he could raise solo? Ya gotta be kiddin' me. Instead of just engineering an easier system for raising the stock mast? Wow.
A good boat that has had something weird like that done to it would likely be fixable. Of course the price of the boat should reflect the repair (although the owner may view his alteration as an improvement LOL).
I would agree that the workmanship and choice of materials looks dubious.
Yes, these are red flags, as they indicate an owner who may have done other bizarre things to the boat that have yet to be discovered. By themselves, they are not indicators that you should run.
Mariners are nice boats, and worthy of being restored. Whether this particular Mariner would work for you depends on price, cost to repair and restore, what your ultimate goal for the boat is, and how good a boat this is aside from the wounds inflicted by the current owner.
Sorry, there just isn't enough info here to tell you whether you should purchase.
P.S.
I wonder what changes he made to the standing and running rigging to accommodate his "mini mast" Different forestay? Backstay? Shrouds? Halyards, etc.. Without meaning any condescension, I would suggest that if you need to inquire here as to whether these alterations are red flags, that you may not have the knowledge to evaluate this particular vessel effectively. This boat could require an expensive (relatively) refit, and might well be a free boat.
