OK. In the first two moves, black must play at least one knight move (as you said a5 is possible, but not a4). But since the bishop is needed at h6 to sacrifice itself on e3, also as soon as possible, I suggest to try the following: 1.b3 Nh6 2.Bb2 Ng4 3.Bf6 gxf6.
The knight could go via f6 to g4 as well, but it had to be Nh6 if the other of the first two moves was a5 and not Ng4.
The problem now is that white has not many moves until the d-file is open. The only 'final' move would be c4, which has to be considered. But that means that the rook (whichever one) may have to make a detour to get to a3. And in any case, the next move would be a tempo consumption move, because the fastest way to open the d-file would be: 1.b3 Nh6 2.Bb2 Ng4 3.Bf6 gxf6 4 ... Bh6 5. ... Be3 6.dxe3.
So, what are the 'best' 4th and 5th move for white? Is it with c4 or something else. One idea I tried was Qc1 - Qb2 and after the d-file opened, Nd2-Nf3 and bringing the a-rook to d1 aiming at a3. But so far not with success.