> 1. Do the other costs scale with the number of panels?

Mostly yes. Once you're at utility-scale, installation and maintainance should scale 1:1 with number of panels. Inverters and balancing systems should also scale 1:1, although you might be able to save a bit here if you're willing to "waste" power during peak insolation.

But think about it this way: If it was possible to reduce non-panel costs by a factor of 5 simply by building 5x larger solar plants, the operating companies would already be doing this. With non-panel costs around 65%, this would result in 65% * (1 - 1/5) = 52% savings and give them a huge advantage over the competition.

> 2. Vertically mounted bifacial PV […] 3. Battery […] costs are dropping

I agree that intra-day fluctuations will be solved by cheaper panels and cheaper batteries, especially once sodium-ion battery costs fall significantly. But I'm specifically talking about seasonal storage here.

> Also, please address the matter of using the overprovisioned power in summer.

I'm quite pessimistic about that. Chemical plants tend to be extremely capital-intensive and quickly become non-profitable if they're effectively idle during half of the year. Underground thermal storage would require huge infrastructure investments into distribution, since most places don't already have district heating.

Sorry, very busy today so I can't go into all details, but I still wanted to give you an answer.