I'm installing 3.25" maple and I have quite a few pieces bent on the tongue/groove side. Some are bent quite a lot, some just 1/16".
Being new to this, I have no idea how much the nailer can straighten boards when a nail is driven in..

I am guessing that I would start from one end and nail towards the other end, and hopefully the nails will pull it in shape.. but I have no idea which boards I can attempt this with and which I should be cutting in half or rejecting.
Is there some rule, like 1/16 is ok, but 1/8 is too much ?

If it has a gap in the middle of the board, I think it will be harder to close the gap, but if the gap is on the ends and tight in the middle it will be easier to close the gap due to leverage?..

I've examined a few hardwood installs, and there are extremely rare occasions that I can squeeze my nail between the planks, so it seems I should be quite careful to avoid any gaps, but in the same time I'm worried that squeezing everything too tightly will not be good for the future?

Yes, start at one end and nail toward the other end. If you have a bowed board use a flat blade screw driver or wood chisel to pry it against the previously nailed row. Pry ahead of the nail gun. The nail gun will help close the gap but the screw driver or chisel will insure that the joint is tight. A board that has been pried straight and nailed and locked in by adjacent rows will stay put. 1/8" is ok to pry into position.

Is there a trick to hold the screw driver and nail in the same time with just one person?

I installed more flooring and got the hang of it pretty good now. Basically the nailer doesn't straighten the boards much at all.. very little.. so if there are obvious gaps in the dry-fit, they will be there unless closed before nailing by some method. I think the nailer only closes something like 1/16 at most

You can either hold the screwdriver pressing against the wood with your knee then use two hands for the nailing, or pull against the flooring with your hand with the nailer in position ahead of time so you only need to tap the nailer with your hammer.