The limits and possibilities of sublingual administration

Factual Questions
1

Am I correct that sublingual administration works by having particles which are small enough to go through the mucous membrane under the tongue and then go straight into the bloodstream?
Drugs, vitamins and minerals can be administered that way. Depending on how much is washed down the mouth before it goes through the membrane under the tongue, it seems that bioavailability of many drugs can be double or tripled compared to GI administration.

What else could be administered that way? Would any drug enter the bloodstream that way? What of, say, coffee? The nutrients in solid food? If someone got blood in their mouth, would it go through the membrane and enter the bloodstream?

What possibilities might there be?

What limits or dangers are there to this form of administration?

2

I think it is more like the substance passes through the lipid layer of the cells of the mucus membrane, and from there into the circulation. The more lipophilic the faster it happens.

You could not get blood cells to pass through, they are far too large. Only substances that can go into solution, not cells.

3

It’s also not all that good, at least, with the stuff I’ve seen. Sublingual B12 seems to have the same effects and time delay whether you swallow it or let it sit for a while under your tongue.