What is the probability of rejection for a long-time-review in the revised version of a manuscript?
In short: We can not tell. There are many influence factors to be considered.

Maybe the following thoughts help a bit:


Usually the same reviewer are evaluating a revised version of a paper. If one of those reviewers is late (due to whatever circumstances), one usually waits for them since they are much more competent in reviewing the revised version then a new reviewer would be.
If they still do not deliver, the paper might be moved to a new reviewer. They again have 8 weeks of time (even though one usually asks for a rapid response).


This sums up to 8 + ~4 + 8 weeks, which is 5 month. Add one month for the editor and you are at 6 month just for the review. With your 18 weeks I would say the paper is within a  time scale in which you don't have to become nervous.

I would recommend writing on your thesis since you did the work and it can be part of the thesis regardless of the acceptance of this specific journal. 

If you don't have a response at the end of January (Christmas break for finishing reviews, Jan. for the editor), I would recommend to politely ask the editor about the status.