How do mRNA vaccines work with respect to presentation of the antigen?
According to a DynaMed article, the egress mechanisms expected to play a role are 3:
(Note that this image may be somewhat misleading, in that [skeletal] muscle fibers [which are very long lived, multi-nucleated cells] don't normally express MHC-I according to a (peer-reviewed) article. I'm unsure if satellite stem cells in the muscle, which serve to repair the multi-nucleated fiber cells by supplying nuclei, have MHC-I.)
As you self-answered, in muscle cells, the MHC-I pathway exposes peptide fragments to "killer T-cells" (CD8)... but there's also release of proteins into extra-cellular space from where they are picked up by B cells, which produce antibodies.
Further MHC-II is engaged in APCs present in muscle and lymph nodes; we know at least from the reported side effects of mRNA vaccines that lymphadenopathy (swelling of lymph nodes) does happen often enough following mRNA vaccine administration.
I think however that the relative ratios of spike protein (following mRNA vaccine administration) reaching these 3 routes haven't been
exactly quantified. (I've search a fair bit, but I could not find any papers on exactly that issue.)
Ultimately, both memory B cells and memory T cells (hopefully) give long-term immune memory against that kind of infection, which at least give the host a head-start in a race against a new infection.
It's worth noting that mRNA vaccines don't produce the same ratios of neutralizing to binding antibodies (to the spike) as a real SARS-CoV-2 infection.
vaccinees generate more non-neutralizing antibodies than COVID-19 survivors resulting in a lower ratio of neutralizing to binding antibodies. These data were already apparent in the early phase clinical trials but remained unrecognized at the time (Walter, 2020).
[...] the burning question of whether the abundant non-neutralizing antibodies do have a protective effect in vivo will need to be elucidated by follow-up studies
mRNA vaccines also produce a lot more (spike) neutralizing antibodies than a real infection (same paper), at least initially--there's apparently a fair bit of decay over time though. Both of these suggest that spikes produced by mRNA vaccines get a lot of B cell attention, which might indicate a lot of them spikes produced by mRNA vaccines egress into extra-cellular space.