What to do with a very long research paper?
Most journals will make any page limits public. Some will take long papers, but that assumes some significance to the result. The best way to find out is to make a submission to one that doesn't clearly state a shorter limit.
Navigating the publishing process isn't co-authorship and shouldn't be offered.
It might be necessary, however, to split the paper into two or more papers, assuming that there are intermediate results worth publishing independently. In applied math, it might be that some intermediate results have their own application. That might be a way to look at it. But the sum of several smaller papers can be expected to total more than the original.
If you make a submission, the editor and reviewers will comment on the writing and if it is &quot;too dense&quot; will suggest edits, though maybe not in specific terms. But you can decrease the density by increasing the length (more explanation) and this, again, leads to the possible necessity of splitting the paper.