12 VAC is enough; 14 VAC is going to produce more heat. But assuming an adequate heat sink and ventilation holes if it’s inside a case I’d think it would be all right.
Half bridge designs are common. See the Frequency Central designs, MFOS design, etc. Ultimately whether it’s good enough depends on your needs.
Your third question: Not sure if you mean a positive and a negative regulator for a ±12 V supply, or two pairs for two ±12 V supplies. But either way, yes. For the first, again see the above mentioned designs, and for the second, see e.g. this.
Fourth question: Right. Grounds should be connected, not power rails.