Ok so i registered here because someone recommended isopropyl alcohol.
this is a terrible idea- the stuff is extremely toxic to your liver. (to give credit where it's due the idea to decarb at way lower temperatures due to oven temp fluctuations is dead on)
if you want to make green dragon or any alcohol based thc extract use ethanol, some grain alcohol stuff (eg everclear). the reason ethanol works at all is because it's slightly longer (CH3CH2OH) and has fewer oxygens than H20, therefore preventing hydrogen bonds from forming with other polar things in the mixture. hence avoid any dilution of h20 possible (95% or better), esp as the boiling point is much higher than ETOH. Also, using something which doesn't have a flame at all to heat might allow you to drive the shit out of the reaction and concentrate the product quite well
second. THC doesn't 'bind' to your pg/vg. thc is a lipid, so you need some sort of lipid-type solvent. so you want something ideally with a lot of carbons and not too many nitrogens and oxygens on it. ideally you want something like butane which is why BHO extraction works so well. your pg/vg mixture is working as a solvent. i can't explain in chemical terms why this seems this way (because VG and PG are isomers) but vg is more 'oily' so at least in theory should work better as a solvent. we need some experimentation here. think of this as making tea with something which is not water-soluble.
aside: if pg is really polyethylene glycol (which is the opposite of what it says on my e liquid in front of me) it would make a terrible solvent for THC due to the amount of oxygens in it- would probably be quite polar.
third. when decarboxylating THC, lechatlier's principle applies- the product of the reaction includes water. so you can safely decarboxylate in any situation where water will be leaving your mixture- which will always occur above the BP of water. I see no reason why 9 minutes at 190F won't work

. THC decarboxylates spontaneously as it dries out over time due to the same reason.
just like any reaction, boiling for a longer period of time will give you better results as you drive the reaction forward. i see no reason not to boil with a thermometer for as long as possible at some temperature well below the combustion of THC.
aside no 2: if you do get separation, the THC will be in the less dense layer (iirc on top)/the one which is more 'oily' and less water like. separation in this sense is a good thing, just remove that layer