I'll see if I can answer your questions.
As for Xanadu...yes, all Xanadus are Thaumatophyllums. If it makes a true trunk and has eyes, as opposed to just having a soft green meristem, and is able to anchor itself upright by adventitious roots and stand alone, it fits the T genus

Philo glanduliferum....I've never seen one, but there are 2 ssp and they depend on the origin of the plant. ssp glanduliferum in endemic to Mexico and Guatemala; ssp camiloanum is endemic to Venezuela.

P. corsinianum is indeed believed to be the oldest Philo hybrid. c1880-1890. This comes up for sale only occasionally, usually in SO FL from what I've seen, I have wanted one for some time (unrealistically, because I do NOT have space for this gigantic plant).

Distantilobun, does not come around all that much. There are some in stock at Natural Selections Exotics in So FL for $75. I never collected it because it really reminds me of Rhaphidophora decursiva

Philodendron brevispathum is truly rare and almost never comes around. When it does its muy expensivefor a teensy plant. There is one for sale at a place I've never heard of before, Frogs Direct (sounds like a terrarium supply, like Black Jungle) for $115.

I have no real idea why Philo warscewiczii green form is so rare. I have had 2 specimens for over 15 years. They were common when I got mine. It may be because they are hard to grow as houseplants, they get pretty large and have to have really sturdy supports

Thank you so much. You definitely answered all my questions. I always have more though. I have two different philodendron crassinervum. One's long and narrow with no margin and I'll send an image but it was an import that lost all its leaves so they are limp and not a great example...
it's just a big chonk with roots in a glass of rain water at the moment. The other is shorter wider blades and they have pink margins. I'll also post an image but it is immature. I can't seem to find much information on them. Neither could the seller of the short blade one, as we are both curious and have researched sporadically over the last few months. The most beneficial finding is a link the seller found which has images depicting both types, it looked to me like there were three types depicted. https://www.exoticrainforest.c...

I have one that I obtained from a friend in Central Florida. He said s an old school aroidist of many years. What was written in the Exotic Rainforest article is true. The species is variable. I'm not sure there are any designated 'forms'. I've never seen one that just has pink albomargination, but, crassinervium is like some hoya…it will color up reddish with a lot of direct light

So mine doesn't get direct sunlight and doesn't get any grow lights. My picture isn't very good but both dark blades that are older have the exact same margin. Just googling images of them I've seen many that are exactly the same. They all have margins and the blades are all more compact. It really seems like there would be something to differentiate them. When I bought the "compact form" I thought I was buying the more elongated blades because I seem to be really obsessed with them! Actually because I don't know the names of many uncommon, older philos I'd love if you relayed any that were similar to these that I have...oblanceolatum/crassinervum, ruizii, insigne, insigne purple, acutifolium, pseudauriculatum, campii/Lynette, three different linnaei, and I have a bi color but it isn't full and nesty like the others. I just got a small variegated wend-imbe and I love it. It's like a mini nest. Lol

This is my crassinervium. I've had it for about 18 months. It's a mess, I should have had it climbing a while ago. Crassinervium is a variable species. I'm really not sure that there are any official (OFFICIAL) 'forms' other than the original type. Some plants are true to type, some aren't. This one isn't.