Boost converter for high side Mosfet
An isolated supply makes things really easy. You can easily get isolated DC-DC converters that will output 12V. By using an isolated DC-DC converter, you can treat the output just like the battery in your circuit. There is less chance for interaction between your isolated drive and the rest of your circuit. Starting at $3-4, it would probably be worth your time for a one-off or low volume solution. 
This is why you do not "simply" connect a boost converter to do the job. You could probably have the input 12V power supply be your source, and then have it run in a discontinuous mode. Voltage regulation of your gate driver supply is a significant concern, as you would probably need something like a zener diode to limit the maximum voltage. Past that, you'd also need a current return path for the boost charging loop, which may be an issue if your load decreases significantly or is disconnected. Even then, I'd probably start from a bootstrap topology and hack in the boost converter.
Your gate driver, when connected to the source of Q1 with an isolated supply, is setting the gate to source voltage directly. It sets the gate-source voltage to either (Vsource + 12V) or (Vsource). Otherwise, you need to know what the source voltage is in order to drive the gate safely. If 36V is applied across your gate-source terminals, your MOSFET will probably be destroyed.
Digikey/Mouser/etc... has a wide assortment of isolated DC-DC converters that generate a fixed output voltage. An "isolated" boost converter does not exist (because it would be called something else). You're probably on your own if you want to make your boost converter idea work.
Yes.