tail ballast antifreeze

I've read the past posts on using ~20% methanol to prevent tail ballast
from freezing. I have these questions three:

1) Is methanol in any way corrosive to rubber parts/seals?

2) Is there a reason not to use ethanol if methanol can't be found?

3) Does a glider flying in the Great Basin area in July, between 15k
and 18k for 7+ hours, really need tail ballast anti-freeze?

~ted/2NO