I am not a plumber and I do not have any answers for you but I would like to know the answers.
Since all traps need to be vented I would think that floor drains would also need to be vented yet I don't think I have seen this done. I have seen a lot of floor drains and many of them were in the middle of a floor and therefore quite some distance from any vent.
I've also seen quite a few trap primers in industrial locations and they all seemed to be discharging a huge amount of water at a constant rate, probably a half a cup a minute. Such a flow rate would be in the neighborhood of fifty gallons a day and that would be a heck of a lot of water in a residence. I sure wouldn't want that in my house. I found that throttling back the flow on these trap primers to just a fast drip would often completely stop in a couple of days thereby defeating the purpose of the trap primer.