I'd prefer to thnk of the various realms as the Great One's dreams, "dead but dreaming" and so forth
Well we know Yharnam probably exists in the real world (though arguably given his phrasing Yharnam could be some kind of oneric realm). We also know the PC gets infused with some Yharnam blood - if you accept the real/dream dichotomy then we know nothing about that blood, the Old Ones could be entirely a dream fabrication and as such the blood could be anything. Similarly all we know is that you signed a "real" contract, its your presumption that its the hunters contract. For all we know it could be medical insurance. Your post does nothing to prove the real/dream theory is "untenable" because, well, it uses information from the "Dream" (which in the real/dream theory is all unreliable) to support it. In essence the "real/dream" theory cant be proven wrong as it were because theres so very little "real" evidence to work with.
Though, apart from not personally liking the "real/dream" theory, its ultimately academic, if nothing that occurs post transfusion is real all that does is change the framing of the lore not the lore itself. We're still left with the same questions more or less.