I am still open to suggestions as to what engines I should use. I will choose one engine to run against, and then choose the next one, so as long as the matches with the first engine has not finished yet, I am still free to choose the others.
My first opponent shall be Pooky 2.7.
Okay, so choosing Pooky as an opponent for this test was a distaster. It seems to have a bug that after about 6 moves in the UCI fen+moves command, it thinks it is white when it is actually black, and thus plays an illegal moves.
So the next one I chose was Yawce (again, based on the observed variance in the normal gauntlet). That one doesn't even support the "edit" command according to Arena, so you can't start from a position other than the starting one.
OK, I fixed the time-control problems in all uMax versions. The problem of the 2'+6" time control you are using is that most of the time comes from the increment. Nominally uMax counts with 40 moves remaining game duration, so the 2' only contribute 3'/move in the beginning, and even later at the end.
And the formula for time/move that uMax was using was basically:
Increment + TimeLeft/(NrMovesLeft+4).
For Increment = 0, this is OK and leaves 5 times the nominal move time for the last move before the time control, which is sufficient to accomodate worst-case fluctuations in the time needed to finish an iteration. In incremental time controls, NrMovesLeft is taken to be 40.
When The contribution from the second term gets very low, because TimeLeft runs out, this formula starts to use the nominal time equal to the increment, meaning that it pretty much plns to use all its time before the axe for the single move, without any safety margin. As the nominal time is supposed to be the average time a move takes, time forfeit follows very quickly. I fixed this by adding a line that replaces the time given by the formula by TimeLeft/5 whenever the formula would give less. This seems to work, I have observed no time losses at 1'+3" so far.
This would make uMax a very suitable test opponent for you: it does support the 'force' and 'edit' commands, allowing you to play from both FEN and PGN initial positions, it is about as deterministic as they come (perhaps even better than Eden), and is very stable. An advantage is that a whole family of uMax engines is available (five of them as compiled downloads from my website), varying in strength from the current level of Eden (uMax 1.6) to the bottom of the CCRL ratinglist (uMax 4.8). So you could upgrade as you go. So you don't have to immediately start with uMax 4.8, which might be a bit too strong, and thus tell you little about the progress that you are making.