maybe try a combo of gaffers tape and food-grade silicone or teflon spray (on the back of the liner). less invasive than making cuts and no damage done if it doesn't work.

i just wear them for a few days on the hill and they're good.

otherwise, hot rice sock method is probably what you want.

i don't bother with the shell at all whatever the old man says.

Did you do a shell fit and if so how much space was there behind your heel? Therein lies the answer.

Bootorials guy addresses this exact scenario.

Unfortunately, this forum won't let me post the link to the vid.

So...google "Stretching liners - Bootorials Ep. 29" and start at 1:15.