1. I have one of my quads as a convertible, but the only experimentation i did so far was to use it as a three fin (thruster setup). This worked well, but overall not as well as the quad setup. SO I can't really say how the board wil work as a single fin. But I can say that as long as you are in any kind of waves, you will probably never use the single fin option. There is one possible exception and this is if you will load the board up with a very big (say 6.0 or something) sail. Then I think a fin setup with the thrusters in front and then a bit bigger rear fin would be a good option. But the extra center box adds about 400-500 grams of weight, I think, so personally I would go without it.
If you sail a lot of flat water and want to tune the board into a more fsw like board, then the single fin makes sense. But only if max speed is what your looking for.
Technically, you will sink if you stand absolutely still. But as long as there is _any_ wind driving you forwards, the board will feel nice and floaty and 5.3 is WELL within what the board handles. ANd if you sail the 99l freestyle with a 7.5 (which is a big sail on that board and that normally requires some good technique) I don't think you will have any problems with the 76 even in light wind 5.3.
But if in doubt, there is always the Quad 81, which is similar in dimension only a bit longer. It fur sure feels a bit "corkier". I think the 81 will handle 4.5 very well for you too, but still I think that you overall would be more happy on the 76 even if you tried them both. With a 5.3 you will not plane earlier with the 81, the only difference will be that slightly floatier feel at standstill.
Look at http://www.hotsailsmaui.com/forum/viewtopic.php?id=2641
for some imags of me )72kg) sailing some small waves in very, very light wind on the Quad 76. I have the 81 too, but I will surely end up with the 76 being the biggest board I need since it works so well even in light wind.