OED has the earliest citation for aby from 1175. The 'buy, purchase, pay for' sense is last cited in 1503 and the 'pay the penalty' sense in 1876, but this is only eight years before that section of the OED I am quoting from was put to bed. There doesn't seem to be any connection with abide which is from bidan, 'to stay'. Shalt is 2nd person singular; shall is used in all other persons and numbers. The only correct linking of shalt with a pronoun is thou shalt. We don't use this much anymore since the singular thou has pretty much been muscled out by the plural you. You might remember this if anyone ever gets on your case for using singular they.