Here is an article I wrote for Scuba Life that I think you might find helpful.
tips-lowly-snorkel
Good article, well worth reading but I disagree with one area mentioned. The article says most freedivers leave the snorkel in their mouth while diving (as I do) but most freedivers (I.e. competitive freedivers, rather that spearos) on this forum dive with snorkel out, that's how they're trained.
The snorkel in/out debate is a long standing one and controversial one. A recent Italian paper made a strong case for keeping the snorkel in but specifically in the context of spearfishing and not competitive freediving.
Also, diving snorkel in, I don't block the snorkel with my tongue, it is unnecessary. But you are right, you need to be careful when surfacing, clearing the snorkel could potentially be enough to cause the diver to black out;. I don't push myself excessively but occasionally I will remove the snorkel from my mouth at the surface, rather than blowing it clear, just in case.
The lower/ back/ under-ear position for the snorkel is not entirely problem free. In chop, when you look-up - for example to get your bearings or determine the location of noisy motorboats - water enters the tube more easily, which can surprise! If you look at the drawings in old snorkeling or spearing books, you'll notice that the diver is usually shown looking down but forward, rather than straight down. In practice, I tend to do both (and, when spearing, looking up from the bottom too) but probably much more looking forward and down. So, having experimented with various options over the years, I currently use a mid-way position, locating the snorkel clip towards the back of the strap but before the rear web of the strap. It's not perfect but seems a reasonable compromise.
BTW Even though I learnt snorkeling from a BASC booklet, I've always worn my snorkel on the right side. Tried left after reading it was more common but never got on with it. Interesting to read the reason for prevalence of left (to facilitate use of SCUBA regulator). Perhaps I once knew that but forgot it. So, right side is ok if you don't SCUBA dive, but left keeps your options open!