Publish a thesis that summarizes papers on arxiv - copyright issues?
All sources are cited appropiately, down to each lemma, but I wonder whether rephrasing could actually lead to getting sued for copyright infringement.


It depends on what you mean by "rephrasing."  If you are genuinely writing in your own words and citing everything appropriately, then there is no problem at all.  Copyright does not protect ideas, just how they are expressed, so writing your own version is not copyright infringement.  Academic norms require citation, but it is certainly OK to write expository papers on other people's research.  Your thesis sounds like a valuable contribution that should be made available to the community, and arxiv.org would be a fine place to do so.

On the other hand, if by "rephrasing" you mean starting with their text and editing it (rearranging a little, changing some wording, making additions, but keeping the underlying framework), then your paper would be considered a derivative work under copyright law.  I've never heard of anyone actually being sued for this in an academic context, but you might be asked to take down your paper, depending on how closely you followed the original papers.  You might also offend the authors if you copy too much from them, even if there's no legal trouble.

The basic question I'd ask is whether you had the other papers (or near-verbatim notes on them) open in front of you while you were writing yours.  If not, then you should be fine, because everything you wrote came from you.  If you wrote your paper while actively studying theirs, then there's more risk: the fact that you needed to constantly look at their writing suggests you hadn't fully internalized their work.  In that case, you should look through your paper and make sure you didn't accidentally paraphrase too closely.