Well, resolved for now. I ended up going with a lazy version of option 2, that's to say I ripped out the second skein and reknit with a different skein that looked lighter and like a better match. I did not rip out any of the first skein to alternate the new second skein in, which was kind of stupid as it wouldn't have been that much more work to do so, but it worked out pretty well. Here we have "before" and after":
I think you can still see a little bit of a line, but not more than you might see in the middle of a skein, so I am trying not to obsess about it! I will definitely alternate rows to transition to all the rest of the skeins but I don't want to alternate throughout the whole thing (although I may change my mind on that, I don't know).
Of course, now I'm worried that I overestimated the stretchiness of this thing and it's going to be too tight, but I'm trying not to obsess about that either.
So, I have come to a bit of a... pause on Vivian. I started the second skein and it is darker than the first. As you can see, I went for about 10 rows in denial before confronting the situation, and I'm just not sure what to do.
I know it is common with hand-dyed yarns for there to be variation, but the skeins looked so well matched I completely forgot about the usual recommendation to alternate rows with different skeins.
1) Rip out the second skein and some of the first skein and reknit by alternating rows and do I alternate every 2 rows or every other row (the latter would require more attention as I'd be working from the WS for some RS rows and vice versa). But won't it just look stripy?
2) Rip out the second skein and some of the first skein and reknit with another lighter skein, also alternating rows (I still can't see a difference between skeins but if you compare strands of yarn at the start of each ball, there is a slight difference)
3) Leave as is but start alternating the other lighter skein in now and hope it will all generally blend in once you have the context and scale of the whole piece
On the one hand, ripping out is really annoying, especially with my precious little available knitting time. It would be even more annoying if I ripped out and it didn't look any better or, heaven forbid, even looked worse.
On the other hand, it would be really annoying if I didn't rip out and continued to be bothered by what to me is a fairly obvious transition.
I knew writing this out would help me decide though, because looking over my options spelled out, I'm thinking of going with option 2. Option 1 doesn't really make sense. It looks like I have possibly two slightly lighter skeins, the first of which I have already used, so it makes sense to put those two together and then gradually blend in the darker skeins.
Any thoughts or suggestions would be appreciated though! I'm probably going to mull this over a little longer.