Shellac is gloss and hard , a bad surface for Latex. Latex does not
bond to hard gloss, did you ever see a gloss primer? no.

Shellac darkens, removing shellac from brick completely will be likely
impossible, brick and mortar are porous . Alcohol is Latex remover also.

Brick is porous, needs no priming. Ive painted maybe 500 gallons on
various concrete and brick surfaces with latex no primer used. No
failures.

The fireplace, if wood was burnt should be acid and soap washed