Anti-matter imbalance allowed it in TMP - is there a fail-safe that prevents it?
Yes. It's called the Navigational Deflector. Under normal circumstances, it'll push things out of your way so you won't collide with it, which is what WOULD have happened in TMP if the deflector had been working properly.
Of course, the Enterprise had dropped out of warp before hitting the debris field, so any objects that were in its path before
it dropped out of warp would have been pushed out of its path anyway.
If so, how did the JJprise warp into a debris field?
Very very quickly.