Have you considered using Aspen alkylate fuel ?

It ain't cheap, but is not supposed to go off. Former work switched over to using the 2T version in chains saws, stone cutters and brush cutters that were always a pain to get going after a period of no use on traditional petrol premix, but were largely problem free on Aspen. As you probably won't use a great deal of it in the mower it may be worth the extra cost if it takes the hassle out of getting it going when you want it.

Nigel B.