The grammatically correct way is "Einstein et al.'s paper", because to indicate possession by a noun phrase in English, the 's should be applied ...
I would say there is a definite distinction between the two, in definition and in proper usage. Et cetera, often shortened to etc., means literally ...
Feb 4, 2011 ... Writers sometimes use the surname of the first author followed by et al. at the first mention of a work that has three, four, or five authors.
Even if you don't speak Latin, you should know the two meanings of et al. Learn what et al. is, how to use it, and what mistakes to avoid.
It is important to use these abbreviations literally and to punctuate them correctly. Many writers confuse "e.g." and "i.e.," and many type "et al." improperly or do ...