Traffic amount concern is the only reason for the hatches...But yeah, single door variant probably is superior (using ramp/float to avoid jamming). If there's enough dwarves to trap one, there's enough dwarves to release one but also to go alongside vampires.
Underwater path would be not troubled by close signal delay, but not sure if it it would work. I'm not exactly clear on how visitors and pools function and whether they will take the shortcut, however (at the very least I might want to include a pool in tavern so that the one in sleeping quarters doesn't magnet all of them or something).
Hm.
This did not get the visitors inside (5/4 trench)
Chopping down the tree in the way got the basic three to move quickly towards zone, taking a dip into the water...And staying there.
I saw one briefly flash aboveground and get back in. Checking their skills with gui/gm-unit, they merrily train swimming. Even when I filled it in to 7/7.
One of them eventually came out of water to ponder medical ethics, then after leaving the next scholar visitor managed to not dip into trench entirely. Ditto for next speardwarf.
After the first two became legendary in the trench, lowered water level back to 4. Expedition leader immediately jumped in:
It also managed to catch next scholar for a bit, till they jumped back out and took the proper path.
Lowering the water to 2/7 did result in the visiting marksdwarves finally leaving the trench, now Legendary +1 Swimmers.
Visitors are weird. Tentative conclusion: swim-only path may or may not trap or redirect visitors. (And vampires may use different pathing entirely.)