For start: I would invite other mappers (including one(s) setting service=) to this thread so they are aware of it.
This is quite tricky and I am not 100% sure…
But if ferry forms part of important road then also loading ramp would be a part of it. So I would tag (in such case) as a highway=primary. And if this does not apply then return road should not be tagged as highway=primary
BTW, splitting it into multiple parallel roads seems a bit dubious to me.
The Mukilteo–Clinton ferry line, like many in Washington State, is part of a state route. It’s a major enough route to be highway=primary or highway=motorway for almost its entire length, other than the ferry approach in Mukilteo. This suggests that the approach should be highway=primary and the ferry line could also be ferry=primary to match. The approach may feel more like a parking lot than a primary road, but the same can be true at any toll plaza or border crossing.