Don't think you'd be able to fully purge the ISO from the VG. This isn't a big concern with EtOH, because its toxicity is low. ISO is a bit more toxic, so you'd want to be sure that it's concentration has been reduced to a safe level before consuming. Don't know how you'd do that in a tincture. It may also negatively affect the flavor.
Never had success with solvent exchange, even when adding VG to the EtOH before evaporating. May have been too concentrated.
Curious why people are cooking their cannabis in EtOH for hours. After extensive tests, found that with QWET, ~90% of activity comes off in the first two three-minute washes at temperatures well below freezing. A third wash produces only about 0.1g-0.2g concentrate (from 30g flower), but it's very dark and has an unpleasant flavor. Anything longer is just going to extract more of these undesirable contaminants.
