Purpose made tile drill. Two types; 1 is tapered, so starts small anyway, the other is hollow diamond-tipped, so full sized but only needed on really hard tiles. Always drill slow. Tip - mask the tile with tough masking tape (the blue stuff) and drill through that to stop the tip of the bit...
Not sure about A trying to luff B. As B overtook, A would have been headed as it moved out of the safe lee bow position, which would add to the suddenness of the heal to windward.
B's fault - you'll have a hell of job to prove otherwise.
How big do you want? FF rig is the same size as an 1940s International 14 - about 150 sq feet. 505 is about the same (ignoring the kite). Of course there's always a lot of used 505 stuff around too.
Inaccurate figures I think. The luff of a Flying fifteen main is 6.25m (20'6"), the mast about 7.5m (say 24'). Apparently the whole rig is a straight copy of one from Uffa's contemporary I14s. 505 rig is almost the same. I guess a GP's mast is quite a bit shorter.
Edit: just checked the class...
Have you done the maths? How realistic is your plan to charge the battery at home every month or so? The tiller pilot and nav lights are both moderately heavy loads. Depends on frequency of use of course.
How will you know when to recharge the battery? It might be worth adding a voltmeter to...