I would not be surprised if I know people who say something like Tessies - it's really common to hypocoristify (it's totally a word) things like that in Scouseland :)

Sainsbury's is definitely a possessive, that's true. Also, something like Selfridge's.

Actually, for M&S, I wonder how much the appearance that Marks is plural influences the Spencer(s) bit...

I might have to rewrite my vowel-final hypothesis a bit :)