Is it possible and allowed to use the 10 hours complex training for the commercial to fulfill other requirements such as the 2 hour night XC or the long XC itself?

Sure, however 10 hrs is only 6 training flights. Most of not all you’ll need to meet commercial standards in the required maneuvers and checkride prep.

Standby, just showed my age.
I’m not sure you still need to do your checkride in a compels airplane.
I think they dropped that requirement.
So yes maybe you’ll have 4hrs left for your 2hr day/night XC.

No problem, just read the regs carefully. When it comes to the multi-engine commercial, not all the aeronautical experience has to be done in a multi-engine, only the specific things that say it within the regulation, in the case of your complex airplane, I'm assuming it's a single-engine-land, which is what the other single-engine commercial requirements require, so of course, it will meet the requirements for both.

Yes you can use the complex plane for any other experience requirements. As was mentioned, do the stuff you must do in the complex first, then if you have time left over, do XCs, etc.

I just passed my commercial in a fixed gear 172 in September.