Solipsism, in this case, refers to placing oneself at the center of things.
So you've stripped Solipsism of all its subtle and specific meaning, yet continue to use the word in this context why - because it adds empty weight to your argument? Reminds me of some guy on this forum whining about "solipsistic drivers" on the road and how absurd and pretentious of a comment it was. Name dropping solipsism in nonsense contexts is practically an ANUS (and affiliate network) meme at this point.
is not the point of rationale the idea that it can be applied to anything?
No, science and rationalism are self-limiting by design. Romanticism rejects rationalism because it ignores "essential truths" in favor of the directly observable - hence its fixation on emotion and the supernatural.
How is subject matter not a valid point of criticism - is the choice of subject matter out of the artist's hands, or does it in and of itself express something?
Because its somewhat divorced from the larger social and intellectual context surrounding a movement. Colors and subject matter are more an issue of personal preference. A major reason "important" pieces of art are famous is because they capture the spirit of the time in which they were made. In metal terms: compare the difference between Thrash and Retro-Thrash.
Why doesn't mundaneness imply superficiality.
Because it forms the bulk of human experience and harbors most of its universal emotions/ideas/"truths". Superficial and boring aren't the same thing. Escapist fantasy art can be extremely superficial.
I find there's much more projection in the two analyses you referenced than in anything I said.
The difference being: the original authors incorporate historical context and various biographical information into their analysis. Anybody can look at a picture and come up with an arbitrary analysis - that doesn't mean it has much weight behind it.