True.
Any additive to a pure PG/VG/Nicotine combo will end up as gunk on the coils because it can't be vaporized. Period. NETs (with all those tobacco particles) and sweet, dark juices are the worst offenders. Sweeteners, artificial and natural colorings and flavorings are all coil killers.
When you shop for ejuices, look at pictures and/or ask questions. Try to pick clean, clear, translucent ones. Halo, Nhaler and Liqua, just to name a few, make such juices.
Not true.
Clean 100% VG is no different from PG. It doesn't leave gunk. I use PG/VG unflavored eliquids to clean my coils.