Good posts so far, I had similar questions.
Some of your questions seemed fairly obvious to me though, I read their website and materials for a while.
As I understand it, OneChicago basically only charges the service of bringing the counterparties together (the fees he linked to), while not being defined as an exchange because they "create" the EFPs from products already existing on the other exchanges. That's my simplistic understanding anyway.
The only recognizable risk I can see in selling EFPs is that the OCC (AA+ rated) defaults while you have an open position. This seems unlikely, but as with any potential defaults (like you compared Spain and Greece) there are ways to inform yourself on the likelyhood of that happening if you do your due dilligence. So I wouldn't say it's "absent credit risk" like you say.
I also don't see a problem with the huge spreads as like he said, if you trade EFPs, you will want to hold them to expiration anyway. The only time you'd want to close positions before expiration is if the OCC is about to default in which case even a 20% spread should be OK.
By transaction costs friction he probably meant transaction costs, where the word friction is just a poetic term to describe decreased profits. I sold some EFPs in small volumes as a test (via ONE) and the costs were as expected, no slippage occured.
As to why there is a 20% spread on EFPs while the underlyings are much tighter, that's an interesting question. I suspect that since mostly institutionals use EFPs, this will depend a lot on the ease of borrowing the underlying stock, and the fact that there aren't as many participants. Market making never was very efficient in any intrest rate instrument, in my opinion, so why would it be in this niche part of the market? Anyway, it doesn't impact EFP trading if you hold to expiration, but I would be interested also to hear a theory as to why the spreads are so different if comparing the synthetic EFPs to the underlying legs.