1) I wish to avoid using another 15V DC supply, either via regulator IC or power supply etc. So can I use a high power 15V zener connected across the +ve 230DCV and the collector of the opto transistor?
Judging from the datasheet for the FET, you can get by with the 5V supply, you have already, in place of the 15V supply. You can't connect the 15v zener as you describe because the FET's source is not referenced to the neutral of the mains. Side note: I am assuming that your +5v supply is derived from a transformer connected to the mains, which isolates its return from the mains, also. If that is not the case, the circuit will not work.
2) Do you or anyone else know that can I also get dimming performance from a reverse phase dimmer for a WIRE wound transformer?
I'm sorry. I don't follow the question.
With regards to the halogen also, at less than 180 deg phase output, the halogen sees pulsating DC (a single polarity) with the circuit. I don't know how halogens hold up to that.