dunno if it's safe but people have been smoking isy wash oil for at least 30 years... your best bet is to ensure all the isy is evaporated as much as possible and only try with a qwik wash honey oil and not a green oil.... what I used to do is after the oil was nice and thick in the bottom of my stainless pan I'd use a hair dryer for about 1/2 hour and heat and blow until the oil resembled shatter, careful pan gets hot hot hot
A lot of herb on the black market has tons of mystery pesticide residue in it anyways. Not really gonna be 100% safe unless you get really regulated stuff. ISO can be completely purged, just need to be more strict if you are afraid of remaining residue. Can even use everclear (95% ethanol) if you really want to be safe.
Health issues aside, make sure you use really clean oil, any impurities will result in carbon buildup in the wick and just not work well after a few hits. Almost all ejuice contains VG, any significant amount of VG in the mix will cause the oils to not homogenize properly, short answer do not use ejuice. peg I hear is questionable, if you want to risk it is up to you, but it will work as long as the oil is clean. Pure PG will work, but the oils will separate after a couple weeks, really you will probably vape it all before it separates anyways. PG is used in about all of ecig ejuice and I would not say it's questionable at all.
You can also try terpenes as a thinner. Terpenes is in weed anyways, so I would not say it is anymore harmful unless you added way more than you should. Terpenes is a solvent so a highly concentrated amount will cause problems.
I was experimenting a lot on THC ejuice before and a lot of solutions sucked ass. Should just get yourself a "wax tank" and vape oils pure.