That too. 
As to this:
I think that the idea has an element of subjunctive. So a) is correct because it’s more subjunctive, not because [some other reasons I don’t comprehend].
It’s expressing, albeit indirectly, his desire to be seen which may or may not occur. In the event it does, he’s happy. That shrieks “subjunctive” to me: assuming facts not yet in evidence.
I readily admit to possibly being utterly full of BS. But that’s how it smells to me.
I think I’m arguing that “him being seen”, in its entirety, is the noun phrase. For comparison, I might say something like “The sign being seen is important for safety”, which might be rephrased “It is important for safety that the sign be seen”.
You do sound unlucky indeed or not as good as I am in picking up context. You could always say “literally, literally” to emphasize the literalness, if you needed a solution, as that is not a stock phrase for indicating an emphatic. As I said, in actual conversation, I literally have not encountered a case where I wasn’t sure what was meant.
So. . . To sum up some of the language discussion in this thread:
People are selectively idiomatic. They use the ones they like and scorn the ones they don’t. Not exactly news.
I didn’t direct that at you.
It is. Any subject in any sentence with the verb “is” is either a pronoun, a noun, or a noun phrase. But that’s not relevant because within it, “being seen” is also a gerund noun phrase. And in formal English, gerunds are usually preceded by possessive pronouns like “his” or possessive determiners like “my”, except sometimes when preceded by a neutral article (“The swimming at the cottage was great this summer”) or a noun, as in your example. It doesn’t change my argument about the use of possessive pronouns. For instance, one might say “Birds singing is a sure sign of spring”. But in pronoun form, it should always be “Their singing is a sure sign of spring” (the possessive) and never "Them singing … "
Both clumsy sentences.
“Having been seen is all that matters to him.”
You wouldn’t write - “Stephen being seen is all that matters to Stephen”. What you care about is the gerund which does not need to repeat the noun referenced later.
No. “A” is correct, “B” is wrong, and your version completely changes the meaning. “His being seen” is present tense, and more precisely, present continuous. Your version is past tense.
If one wants to modify “A”, it’s sometimes acceptable to drop the possessive pronoun altogether, since “being seen” is an acceptable noun phrase just by itself.
Well I agree. It goes beyond my point and makes for a better sentence.
Or, American Spock: “It does compute”
It’s a gerund phrase, acting as a noun. “He is being seen” would be present continuous.
“Him being seen” is wrong in traditional usage, but colloquially and casually it’s common enough to be okay. (IMHO).
I’ll suggest we can almost always find a way to say something more sloppily and ambiguously while still giving at least the general gist of some meaning. But labeling that a virtue is IMO a mistake.
But … that’s all the precisely most people think or speak. And most people don’t write like writers. They write what they’d otherwise say, which is what they’re thinking.
I greatly prefer precision of expression. But when coupled only to vague nods in the general direction of thought, we get the opposite of the old aphorism. Not this:
Measure it with a micrometer, mark it with a crayon, and cut it with an axe.
but instead this:
Measure it by eye, mark it with a rattle can, and cut it with a laser beam.
I think I think of ‘invaluable’ more in the older sense of ‘not capable of being valued’. Similar to ‘priceless’- so precious and one of a kind that it’s impossible to assign a price or value to it.
Context and balance matters.
Are they writing conversationally in a casual email or social media context or a trying to persuade a board to support a particular course? Other? One calls for much more time composing and revising than the other.
For the context, how useful is conforming to a specific standard style versus the value having your own “voice” heard, which possibly facilitates better connection and persuasion as well?
I would be so delusional as to imagine myself a skillful writer but even I understand that writing with the target audience and purpose in mind is nearly on par in importance with having something worthwhile to say.
Agree w @solost. Continuing from their point …
An aspect of both “invaluable” and “priceless” is that the thing is in fact quite valuable. Not necessarily quantifiably so, but still of great value.
It’s hard to put a dollar value on e.g. fallen tree leaves, but nobody would label them “invaluable” or “priceless” .
Compare and contrast with “inestimable”.
It’s also hard to put a dollar value on having my kids rake up those leaves, being difficult to estimate the value of a theoretical construct that has never been observed to actually happen 
I wish people did that. So often, I’ve had a student come to me for help with an assignment, and say “I want to say <something>, but I don’t know how to say that”, to which I’d reply “You just did. Write that down.”
Makes me think of the Mastercard commercials:
Pile of leaves in the yard: Free
Price of kids being paid to rake them up: $10
The indoor silence while the little blighters are out there making an even bigger mess: Priceless
I think this is a very astute observation, and is arguably one of the most important reasons for learning standard English. One might say that, depending on one’s career and aspirations, having a good grasp of formal English is nearly on par in importance as knowing the rudimentary basics of the language in the first place.
I’ve mentioned before that linguists who defend sloppy colloquialisms and other misuses of the English language invariably do so in impeccable language. As someone pointed out, otherwise they’d have no credibility. If, for instance, you want to defend the use of “ain’t”, you had better do it from a reasoned and learned academic perspective; you won’t get anywhere claiming that “there ain’t nuthin’ wrong with sayin’ ‘ain’t’”.
Those who dismiss the value of promoting strong skills in writing and speaking by deeming it “classicist”, and the domain of “elites” and the “ruling classes”, would do well to remember the importance of speaking to your target audience in their language, in their own linguistic register. And if you want to get anything consequential done in this world, this is precisely the group that you may often have to appeal to.
Yet we often hear complaints about language use that is completely appropriate for the context and the audience, is in the linguistic register of the bulk of the target audience. Members of the “elite” class sometimes have a difficult time understanding that the context is not one for which they are the main audience of interest!
It is also not a great idea to try too hard to speak a target audience language. I, for example, should speak in language understandable to today’s teenagers, but my using the slang that is their language would not work well. IOW don’t dismiss the balance part, the need to also maintain authenticity, a real voice.