I would personally allow liquid lynchpins, but they wouldn't work any differently than any other lynchpins. You could splash a target with one and then set off the spell, but it wouldn't happen any differently than putting a sticky note on the target before triggering the spell.

"Health potions" wouldn't need to be drunk, just in physical contact with the target, so splashing yourself with the stuff would be all you need to do. In fact, I might say that the magical ingredients necessary to make the liquid a lynchpin would wind up making the potion poisonous to ingest.

The big question becomes what constitutes enough damage to a liquid to remove its magical properties. That becomes a real fuzzy area, and is probably the biggest concern of liquid lynchpins. I would say dispersing to a large enough degree would count. How much is enough I would have to decided on a case by case basis, but putting it into something like a neurostun grenade to scatter it over the whole room would definitely be crossing the line.

As for your specific example of writing on the wall, I would say that if the liquid was not enchanted and your method for manipulating the lynchpin was to scribble with it over the wall, then yeah, I would probably allow it. But now you've got to spend a number of minutes doing that equal to the force of the spell, then take drain, and then have a preparation that's not going to move until you set it off, and it's going to be much more fragile than a solid object as the lynchpin.

For your other example, page 306 describes line of sight rules for preparations, and that would mean that it targets who the magician wants if it is a command trigger, or the closest target if one of the other triggers. I assume with the non-command triggers it would use the LOS of the preparation because one of the big advantages (perhaps the only advantage) of the other triggers is to not be there when it goes off.