Yes, most (i think all) lazy DFA engines have a mutable DFA behind a lock internally that grows during matching.

Multithreading is generally a non-issue, you just wrap the function that creates the state behind a lock/mutex, this is usually the default.

The subtle denial of service part is interesting, i haven't thought of it before. Yes this is possible. For security-critical uses i would compile the full DFA ahead of time - the memory cost may be painful but this completely removes the chance of anything going wrong.

There are valid arguments to switch from DFA to NFA with large state spaces, but RE# intentionally does not switch to a NFA and capitalizes on reducing the DFA memory costs instead (eg. minterm compression in the post, algebraic simplifications in the paper).

The problem with going from DFA to NFA for large state spaces is that this makes the match time performance fall off a cliff - something like going from 1GB/s to 1KB/s as we also show in the benchmarks in the paper.

As for eviction techniques i have not researched this, the simplest thing to do is just completely reset the instance and rebuild past a certain size, but likely there is a better way.