What is the longest discrepancy between death and publication?
Radcliffe, J. M. "Some properties of coherent spin states." Journal of Physics A: General Physics 4.3 (1971): 313
was submitted in 1970, published in 1971, and contains the following:
At the time of writing this post, the paper has 951 Google citations.
There is no hint as to the date of the passing of Prof. Radcliffe but it is the only publication by this author on MathSciNet; it is entirely possible that Prof. Radcliffe could have published in Journals not covered by MathSciNet. The Mathematical Genealogy project contains references to a single John Radcliffe who graduated from University of London in 1967, but the general area of the thesis and the aforementioned work do not match very well.
Anyways, this is clearly an example of posthumous publication not done in collaboration.
Yes, papers get published posthumously.
I don't know who the world record holders are, but Klaus Schulten, one of the greats of biomolecular simulation, who passed away in October 2016, has written several papers in 2019 (according to Google Scholar). These include a PNAS: https://www.pnas.org/content/116/12/5356.short