You really need to speak with a lawyer directly to try to get exact answers on what is going on; I'll answer as best as I can based on what information is here:
1) Very good question - it makes no sense for them to do this absent a showing that you committed fraud in first getting conditional permanent residence. Your ex may have written to USCIS and made some form of accusation. No real way to know.
2) No - not right now. terminated means they're over, "without prejudice" means that it wasn't a decision on the merits and ICE can bring the case again later. Most likely, if the I-751 is denied.
3) If they did formally terminate your conditional permanent residence status, then there is nothing to extend. HOWEVER, approved of an I-751 would still grant unconditional permanent residence status.
4) I wouldn't until this is cleared up.
There is relatively recent guidance - a few months back - that removal proceedings should be terminated this way when an agency adjudication (such as USCIS determining your I-751) might solve the problem (I'm paraphrasing a bit).
Speak with an attorney, bringing all of your paperwork, to try to figure this out.
If the judge terminated the case without prejudice, it could be for many reasons, but the result is that you are not in removal proceedings.
The answers to your other questions depends on why the judge terminated the case. Without prejudice does not mean they are prevented from putting you into removal proceedings again.