George,
the easiest indication that helps to check where a paper comes from is the datasheet. Compare the recommended filtration settings, they are usually identical to that of the OEM.
"I've also heard (elsewhere) that Tetenal's RC papers are rebranded Agfa and/or Ilford"
Yep.TT Vario Ultra is Agfa MCP, TT Vario Comfort is Ilford MG4. Any difference you may notice in processing is within batch variations.
The datasheets given by Tetenal proof this (and it was confirmed by a TT employee and a german retailer).
"I even once heard (although I shouldn't be sayng this, it was "classified information") that Ilford was manufacturing Agfa's multicontrast RC paper..."
I don´t think so. Agfa coats the MCP more or less 24/7 on a big line and was able to cope with demand when the Ilford plant was offline.
"I would be interested to know whether Maco and Wephota of Germany make their own paper, for example."
Nope. Both buy from a number of suppliers like Forte, Fotokemika, Agfa.
Wephota does a little bit more - they buy bulk master rolls and cut by themselves, they are placed in the former Tura plant in Wernigerode which has been used by Orwo in socialist times - it was a film coating plant (disfunct now) and a confectioning line.
"I understand that talk like this might offend the manufacturer of the product in question,"
I see no reason for that - the manufacturer rarely ever is offended, it´s usually the distributor, the guy just putting his label on it and trying to pull a marketing stunt by promising lot and demanding premium prices.