I did see on person say that heating it up makes the final finish more
brittle. But since only one person has said that, I'm choosing not to
believe it for now

Shellac has organic acid components. Over time, the acids react with alcohol
and form esters --- which won't dry properly. I'd guess that heating the
mixture would just speed up the esterification recation.

I'd suggest to just mix the flakes and alcohol and shake it occasionally. I'd
should dissolve in a day. Also, have you considered the alcohol? Maybe it
contains too much water. Try again with different alcohol and see if that
works.

Joel Jacobson