Question 1; is there any thermal comparable to the 233 price and feature wise available today?
Answer 1: Yes! Take a look at the Bering Optics Hogster models on the Night Goggles website

Question 2; what’s a good NV clip-on that can be mounted behind an eotech or aimpoint.
Answer 2: The PVS-14 can be mounted behind both. You will need a weapon mount such as the TNVC TM-14 Mk3 and the optics must be night vision compatible models. Weapon mounting the -14 isn't the best way to utilize it. It's best head mounted and used with an infrared aiming laser.

Question 3; can you use a prism scope such as a 4x acog with clip-on NV?
Answer 3: NV compatible EOTech or Aimpoint is what you want for NV use. You can use your head mounted NV to aim looking through them for passive aiming however like mentioned in #2 a laser is more effective.

Both. You use a laser pointer (the dot) to aim. Some units such as the ATPIAL-C, DBAL D2, MAWL C1+ also have a built in IR illuminator (the D2 is LED) and a visible pointer that is slaved to the IR pointer. The slaved visible laser is very useful for zeroing and confirming zero without needing to do it under NV. The illuminator aids you by illuminating the target and eliminating bloom from the pointer. Although night vision is an amazing capability, shadows, crevices, light sources, etc. can diminish it's effectiveness and you need to provide additional IR light. There are settings to use the pointers only, pointer + illuminator, or illuminator only for aiming through a night vision compatible red dot or holo sight.

Which is better, head or weapon mounted, depends on exactly what you are doing with it (long distance precision, hunting, SHTF, etc.). If you can only choose one or the other (head or weapon mounted), I recommend head mounted. It offers the most versatility and you won't be pointing your gun at everything just to see in the dark.