It certainly is provable, though a brute force approach would be time-consuming.
There are 11 * 2 * 9! = 7,983,360 unique arrangements of pegs. And, if I counted correctly, there are 68,600 unique solutions for peg #1, with five rotations of each solution for each arrangement of pegs. That's 343,000 positions to check for each arrangement of pegs. More than 2.7 * 1012 positions to check, and we still have 15 more triangles to place.
Should we go ahead and reserve time on some super-computer?
BTW, the 12th triangular number is 78, not 45.