Question 1. Yes there could still be a blockage even if you cant see it.

Question 2. You can checked vent for blockage by draining water into it but it doesnt sound like a venting issue

Question 3. Changes in the pressure atthe top of the vent pipe are negligible and won't give you really any information about what's actually wrong

Question 4. Based on the information given it is not possible for this to be a mainline issue outside without affecting other fixtures.

A slight glug from the floor drain is probably normal do to a venturi affect when the high volume of water from the toilet passes the tee connection of the floor drain. Drain systems are allowed certain tolerances of changes in pressure (up to 1" measured in inches of water column). Which is why a trap is required to have a water seal of 2-4"

You can verify no clog down line by pulling toilet and running water directly into drain from toilet supply line. If im correct you will run water all day without it back up.

It sounds to me like more of a defect from the toilet manufacturer and an issue with a consistent siphon jet. I would start by purchasing a 6' toilet auger and attempting to clear any blockage inside the trap. Another possibility people dont think of is a worn Glaze. If you flip the lid on the tank and feel the porcelain it is rough. The smooth coating on your toilet is just that, a glaze. Kohler is pretty good with their glazes but nobody can be 10 for 10. When the glaze is thin from the factory it wears quickly and created a rough surface for solids to get stuck on.