Yes, it should be possible to prise a shift key off carefully without doing any damage.  Keytops are never moulded directly onto the keylevers.  They are always pressed into place when the typewriter is assembled at the factory.  I have never made a replacement keytop by moulding as you describe, but I am sure it can be done.  Other alternative ideas might be to obtain a block of plastic and hacksaw, file and polish a new keytop from that.  To produce the slot underneath for the keystem, you could chain-drill a series of holes with a small drill-bit, then heat the keystem and push the new keytop down into place, thus part-melting the plastic and producing a perfectly-moulded slot.  Possibly the best and easiest solution would be to take a similarly shaped keytop from a valueless broken 1980's portable and adapt that.  OK it wouldn't be the same shape, but maybe a black one wouldn't look too bad (assuming you have the green keyboard).