In terms of "weak onscreen evidence", I trust a computer screen or two in the early movies would have reproduced Franz Joseph's related graphics, which designate the forward domes as "sinks" for a sink/source type drive system. Alas, no such readouts have been identified as having made it all the way to the movie screen so far, and the text would be illegible anyway.
In the otherwise complete lack of direct evidence, I'm forced to point towards the very direct dialogue evidence ("Samaritan Snare", ST:INS) that the identical-looking features in the TNG era are indeed Bussard ramscoops. What such ramscoops actually do
is a different matter, and onscreen evidence on that is extremely vague in good and ill. We know the scoops can blow out hydrogen and suck in dense volatile gases from a nebula, but both appear to be exceptional procedures going against the design intent of the devices.
Since the red domes are not a mandatory component of starships and shuttles, but are only found on about 80% of them, I'm tempted rule out a fundamental role in warp drive outright. However, it is curious that all warp nacelles in Federation ships seem to be installed so that their forward ends protrude, even when other parts of the nacelle may be quite extensively covered or buried (say, the Defiant
classes. Indeed, if nacelles are buried or turned into cowlings, all the more effort is made to establish red domes at where the forward ends should be.
Hence, I'd argue that we can trust that
a) the devices are called Bussard ramscoops for certain, even on Kirk's ship (or that's what Riker would call them on Kirk's ship even if 23rd century terminology differed)
b) they thus probably "ram" and "scoop" something, although not necessarily the way Robert Bussard originally intended,
c) they absolutely need to be located right in front of the warp coils for that, and
d) while they don't need to glow red, or be particularly large, they nevertheless are vital for warp travel.
For the latter argument, let's look at the visuals. Many of the cheaper TNG nacelle solutions (marker pens on miniatures, hexagon shapes on shuttlepods) have relatively tiny forward ends that never exhibit a glow. The TMP style of warp nacelle has narrow grooves but does exhibit a red glow in some DS9 visuals of the Miranda
class. And when the E-D shuts down warp power generation for dilithium maintenance in "Skin of Evil", the red ramscoops go dark. This does not happen in connection with emergency plasma venting such as in "Eye of the Beholder", or when Kirk's ship goes to silent mode in "Balance of Terror" in TOS - but the domes do become a bit dimmer in the TOS-R version of those events...
It wouldn't be difficult to claim that the intensity of the glow is a factor of energy levels, then, and that the ships and craft that seem to lack the glow might be running the system at low energy levels. Would that make it a non-vital system, when Kirk in the movies manages to avoid cranking it up? Perhaps we could actually argue it's a nice-to-have extra on basis of this very fact: ships don't need to run the system hot, but modern ships can afford to even when they don't need to.
The four arguments (or the first three already) would seem to rule out your favorite #2, because it's difficult to reconcile that with the name "Bussard ramscoop". However, a deflector or "snowplow" system could very well be called exactly that, and it might be that warp coils are extremely sensitive to particle bombardment when actively messing with subspace, whereas the rest of the ship is not particularly sensitive. Antimatter generation or warp propulsion could make use of "scooping" as well, but I'd rule out #3 because antimatter could be created anywhere (and is known to be stored elsewhere on Kirk's ship, too) yet the red domes are invariably in front of the warp coils. #5 I'd rule out on the same basis.
My clinging onto the rather blatant connection between TNG dialogue and cross-series visual unity thus drives me into favoring either #1 or #4, with #1 the subjective winner.