Messed up my first project (HP-97)

[Hans Brueggemann]

imho, your calculator doesn't look that bad. it just seems that the IPA treatment has partially dissolved the top matte varnish of the keyboard, hence the white residue.
the objective now is to remove all the remaining varnish and residue without deteriorating the imprints. remove all keys from the keyboard and clean it. then, protect the surrounding from overspray with tape. give it a couple of very thin fresh coats of matte varnish from a spray can (or airbrush). if a layer of varnish gets glossy while spraying, you are overdoing it. test your spraying technique on some black scrap plastic first. let the varnish thoroughly dry between coats. if done properly, the keyboard faceplate will look like new.
Don't use a heatgun, it will just ruin your calculator as the plastic might warp sooner than you would expect.