My understanding is that the mast needs to be fixed at the heel, so that it can not rotate - and so that it can not jump out of the mast step, or even worse and punch through the bilge, due to violent pitching (as happened to David Lewis down here, many years ago).
If you attend to that properly, and its fairly simple, how then can the mast ever move due to rolling, or even capsizing?
At the partner, or deck level, the mast needs lateral support - the partner/deck structure must be strong - and a detail to close the gap and prevent water leaking in such as spartite (or equivalent) - or wedges and a mast boot as described by Arne (or equivalent) - and nothing else except robustness. This also is the area where the mast needs to be at its strongest in regard to bending.
Sometimes people have had to carefully work around a situation where holes have already been drilled in a mast tube, but you would want to think long and hard before doing it on purpose. Bolts or welds are not a "100% solution" where no fitting is called for - of all places, especially at the partners. At best they will add nothing to a mast which is already held in place perfectly well at the heel - and at worst are a quite possible cause of failure.
With a little boat like Karl's things are a bit less critical and that sort of creativity is probably OK - but it seems to me that what you have been considering for your Contessa is the result of over-thinking and imagining too much, while ignoring an existing well-proven body of thought.
A Contessa with a split junk rig sounds like an exciting combination and I sure wish you were here ("down under") so we could see a match race with Zane's Pango (which sails beautifully). Your project is of great interest - best wishes and keep us informed.