What can I do if I don’t completely understand a paper?

Ask on mathoverflow!

It's a site like this, but for math at research level. There is incredible expertise in all fields of math (I know of) over there. Before you ask, look at "How to ask" and also take a look around at the site to get a feel for how the folks roll.

Should you submit your paper under this situation - possibly.

People assumed that Fermat's last theorem was true for centuries before it was proved. You don't necessarily need every detail of every proof.

If it seems reasonable that it is true then it's okay so long as you are clear on this assumption, and caveats thereof, and your paper is essentially going to be retracted if the inequality isn't true, or have conditions to it.

From your description however There are quite a lot misprints and somewhat mistakes this tells me that the paper is of questionable if not poor quality, and your mistake was doing further work that depended on this. In light of this I would not wish to publish without at least some verification of the inequality. Peer review in no way means something is true.