You can get garnet colored shellac flake from shellac.net or from Lee Valley. The Russians used de-waxed garnet shellac originally, not stained shellac.
Just apply the new coat over the old coat. Lightly knock back the shine slightly with fine steel wool after the last coat has dried for 24 hrs.
No prep required.
Shellac is used over oilly wood all the time as a barrier coat before applying varnish. It's designed to handle that.
To make shellac from flake, desolve it in denatured alcohol according to the instructions.