No! Once your chocolate is in temper you have dissolved the unstable crystals and you have a predominance of stable beta crystals in your chocolate. As your chocolate cools, it will become thicker as the beta crystals grow and multiply. As it thickens you can warm it again either by heating with the heat gun or microwave, or by adding warm untempered chocolate. The beta crystals in the tempered chocolate will act as a seed to temper the newly added untempered chocolate.

You will be able at this point to heat it a degree or two higher than the standard working temperature in order to make it thin enough to work, without driving off all the beta crystals. Just be careful that you don't exceed about 34 degrees for dark chocolate, 32 degrees for milk chocolate and about 31 degrees for white chocolate. 

Yes. If you have chocolate left over after a session of molding you can re-temper it. Or if you have chocolate that you improperly tempered and it is streaky or spotty you do not need to discard it. Heat and cool this used chocolate just as you did the first time to bring it into temper but add about 10% new chocolate for best results. This will not work however for chocolate that has been contaminated by dipping things into it. Leftover chocolate used for dipping is best reused in a ganache or a bark.