>> About the mast, I was thinking that since the gennaker is pulling in the top, it would maybe cause more stresses than if the forces would be evenly distributed.<<

I have no idea what Im talking about, so given that proviso let me rush right in and look like a fool :)

there were spinnakers that gave lift as well as forward force, but you still need a block on the mast top
but it does take some of the strain off the mast

cat ketches run wing on wing, given that technique
boom code zero's out wing on wing from each mast

or perhaps a new sail is required, with a fibreglass spar in a pocket as a batten to increase the head of the sails height

the reality is, a cruiser rig on unstayed masts isnt going to be wholly suitable as its an anathema to racings cause. Perhaps the best compromise is the square topped cat schooner rig that is so oversize it is most commonly reefed


Now let me help you out and you can begin with "What the...?  "