M-Y
1) Yes, but we try to run like jars in the same batch if we can - they stack more uniformly and reduce accidental breakage
2) Start with clean, sterilized jars, of course. Heat the lids b-4 using to soften the seal. Make sure your jar lips are wiped clean.
3) Yes. If it is windy, you might have to put up a windbreak. It does require you fuss with it a little more than on stovetop, but I sure wouldn't can fish in the house
4) Dunno. USDA might have a page on canning.
5) One last thing, to reduce breakage, start with a cold canner, cold jars. When the cook time is over, don't remove the weight until the pressure drops to zero or close to it. Sudden changes in pressure will cause the jars to burst or not seal properly.
6) Try adding onion, shallot, garlic or peppers to your tuna prior to canning. Just throw a piece of whatever in there. Mmmmm...