See my update at the bottom of the question
When installing my 4 bolt Shimano 105 FC 5800 chainrings on a modified FSA Gossammer Crank (TEAM ZWATT ZPIDER) I first test fitted the chainrings with some chainring bolts I thought were compatible (they fit just fine into the threads, but theyre a little bit too short in hindsight).
I need to mention that the chainring bolts thread into the outer chainring directly.
I then put the chainrings on and screwed the chainring bolts into the chainring, not very tight but "hand tight".
The bolts were left there over night and aftering the correct ones this morning, I cannot get the bolts out.
I stripped two bolts trying to get them out but they just wouldnt budge, so now I'm left with a crank with two stripped bolts.
Now: the chainrings are somewhat disposable (I can order new ones but) but the crank is not, so I played with the thought of just sanding/drilling the chainrings out of the crank and this time mount correct parts. But do I have any other options at this point ?
I have sanded down the securing part of the chainring bolt using a dremel, at the cost of scratching and ruining the inner chainring.
Since the bolts were not per se stuck but have rather dug themselves into the inner chainring they sat quite loose in the outer chainring and could be undone by hand.
But in case they were stuck I believe the protruding part of the screw can be grabbed with a pair of pliers and be used to remove the rest of the screw from the outer chainring.
The cost of the whole operation was one set of chainring bolts and the inner chainring.