How did they gain access to Oldman's 'safe room'...

In addition, Claire would have had to have exceptional observation and recall abilities.

Virgil faces his glove closet shelving (gloves on)
Shelving splits open horizontally with remote
LCD pad #1 -> presses button in lower left of number pad
LCD pad #2 -> 2 button columns, presses upper left button
combo: 6, 2, 4, 9 ->OK

All done so quickly that I had to put my viewer in slo-mo to catch it all.

True, but that wouldn't reflect the type of people they were. They prided themselves on making away with valuable things undetected, constantly teetering between the lines of what's real and what's forged, so to have his team force their way in would have been too messy and out of character. And that was my point, most thieves have mutual profit, but since Virgil held on to the paintings that were worth millions, while giving Billy only a small fraction, it simply was an unfair deal to him.

I guess the main reasons why I didn't see his artwork being rejected by Virgil as his driving motivation was because I felt as though even if Virgil DID believe in his work, he would have still gone through with the con, since he knew the value of what he could get from Virgil. That's just the type of person I saw him to be, given their relationship.

Surely each member of the plan was payed handsomely, and I can agree that maybe Billy's motivation was more personal than the others were, but all together, I saw it just as an elaborate job first, to make away with some of the world's most valuable paintings, and then second, to psychologically ruin Virgil during the aftermath of his discovery as his payback for not believing in him. I just personally saw the psychological aspect of ruining Virgil as secondary, and his painting of Claire's mother, both being used as a way to sell the con first, and then later, used simply as his calling card.