Had to reuse a compression collar and nut

How bad is this? I had to replace a tankless hot water heater. I ended up getting the exact same one, and the pipes are pretty much in the same space, close enough to get the nut on, although there is a tad bit of torqueing of the copper pipes ones its snug. The unit is off kilter about 4 degrees, which I think is fine.

The old unit had a little stress on teh copper pipes as well...as in they aren't perfectly square but the unit is....I guess this is OK.

Also, I had to reuse the compression nut since I can't get it off. It has been hooked up under pressure for an hour and it is not leaking yet.

Will it be OK most likely?

Also, this new unit gets MUCH hotter than the old. When its done being used, you can feel the heat emanating from it a foot away. My infrared thermometer shows the chambers at 270 degrees. How can that be good? When the water shuts off, after a few seconds, you can hear the water bubbling and gurgling in the unit...sounds aweful.

Tips advice? Is it OK? Should I worry?

You'll probably be fine with the compression fittings. Other than that, I can't help, little experience with that type of unit so I can't say what is normal. Note, if you don't delime (remove mineral deposits) in a tankless unit periodically, it can severely reduce its ability to transfer heat after time. On a new unit, with everything pristine, I'd expect the transfer to be as good as it gets, and only go downhill from there, unless you perform that maintenance (often recommended at once a year). If the unit doesn't turn off promptly when the flow stops, it could get messy. There is some residual heat the heat exchanger, so going from full bore to off, depending on the thermal mass, you might expect some boiling.

Theoretically, the more heat transferred to the water, the cooler the unit will run. Any heat NOT used to heat water is wasted, either up the flue or into the room.