Isn't this a conflict of interest and self-dealing on the part of CEA/EVF leadership?

Claire Zabel confirms in this comment that

Open Phil gave most of the funding for the purchase of Wytham Abbey (a small part of the costs were also committed by Owen and his wife, as a signal of “skin in the game”)

There are two things here that seem to be true, either of which I think it's reasonable to object to (though my own views are not yet settled):

1. FTXFF and EVF had shared board members, and FTXFF gave a bunch of money to EVF.
2. OpenPhil also gave a bunch of money to EVF, to buy a fancy house to use as an event venue.

You're persistently trying to link the two of these together in a way that implies...something nefarious, I'm not sure exactly what. Currently they seem to me to be two largely separate events. You need to elaborate on the chain of reasoning here, because right now I'm just confused.

I don't see any deep problem with a funder saying to Will and Nick, "I trust your judgment, so please help guide the Future Fund, including by disbursing however much you think is needed or ideal for your existing orgs (CEA/EVF)."  It's only a conflict of interests if the funder doesn't really trust them (or has different philanthropic priorities), right?

Of course, often we don't trust people, so we put up conflict of interest rules.  But it's not clear that such rules would have been desirable in this case.  After all, there would seem obvious reasons for SBF to both (i) want Will's & Nick's guidance for the Future Fund, and (ii) want to support CEA's work.

Yes, but conflicts of interest are often waivable -- and I think this one would be. The risk was that MacAskill et al. would not impartially carry out their roles at FTXFF because of their involvement with EVF. If FTXFF (and its donors, who were by and large also its board members) wanted to waive that conflict, then that was fine. It and its donors were the only parties who might not be getting the conflict-free advice they were entitled to.

I don't have any thoughts at the moment as to whether this was a good idea, but I don't think "conflict of interest" or "self-dealing" is quite the right paradigm here.

Moreover, did CEA/EVF leadership make the decision to purchase Wytham Abbey because they knew they'd be getting enough money from FTX to make up for the cost?

The answer to this is a straightforward "yes, albeit indirectly", if you look at the comment they made on the subject:

At the time we committed the funding (November 2021, though the purchase wasn’t completed until April 2022), there was a lot more apparent funding available than there is today, both from Open Phil and from the Future Fund

...

With the huge decline in available funds since November 2021, I don’t know whether we’d make this grant again today

Essentially, they thought they were flush with cash from ftx, so they could afford to be more reckless with grant projects, even if they weren't funded directly from ftx. 

The comment also answers your question about the funding source:

Hey, I wanted to clarify that Open Phil gave most of the funding for the purchase of Wytham Abbey (a small part of the costs were also committed by Owen and his wife, as a signal of “skin in the game”)

I don't know how long major housing purchases take, but 5 months doesn't sound too unreasonable. Overall the manor purchase seems like a pretty bad idea, but seems unrelated to conflicts of interest. 

On the other hand, I completely agree  with your concern about Will being on the board of both the group giving out the funds and of the groups receiving the funds. I think this type of behavior, even if meant well, will both cause results to be obviously biased, and provide a huge incentive for bad actors to exploit the movement.