I suppose I am late with the summary of how it worked out in the end. Well, I reached an impasse. I can either have no dimpling, but the prints will delaminate when I bend the mount board severely at corners, or they will have good adhesion at the price of some edge-long dimples. I think I made the dimples less visible, but I cannot get rid of them entirely. Looking just now at 28 mounted prints I can see that only 6 have almost no dimpling (the edges ever so slightly come up though), while 22 have dimpling. As I have been making notes on the times in the press for each of those, I can only see that what is common to all of the less dimpled prints is that they all (bar one) delaminated after an initial pressing and had to be returned to the press for extra time and overall they were pressed at a lower temperature for longer.
I have been cooling each print between two sheets of glass immediately after pressing for a minute or two, then cooling a little more between mountboards, and testing for adhesion by corner bending.
Maybe I am bending them too much?
I am, overall, at a loss to make sense out of the whole "experiment". I mounted some 40 prints in total. I started with lower temp (80-90˚C, 175-195F below the top protective mountboard, 100˚C, 215F at the platen) and longer times 3-3.5 min. This has led to less edge-long dimpling but far worse adhesion with frequent delaminations at corners, requiring a print to be returned once or twice back to the press. One print went back 3 times before it passed the "bending" test.
Then I decided to increase the temperature progressively at the platen to 121˚C (250F), which was giving temperature of about 90-100˚C (195-215F) under the top protective mountboard (4-ply all the time). Trying a sequence of progressively shorter times clearly failed to activate the Unimount Classic tissue evenly at 45 s pressing time. Time of 1 min 5 s seemed to do the trick, with prints adhering well or requiring one more trip to the press for another 1 min, when they always seemed to adhere. Unfortunately, the dimple was back, perhaps not looking quite like a "valley" but more like gently lifting edges.
I don't know what to try next, but I am keen to try a different tissue to see how it may improve things. I wonder if I can get any ColorMount in Europe, or perhaps I will try one of the HotPress's Archival Grade ones, they are reversible like Seal's BufferMount.
I have also tried other suggestions made here, that is pressing "upside down" with the print under the mount board, the press had been well prewarmed for over a day, and I have been using pre-dried boards and prints.
If anyone wishes to make any other suggestions, I would be grateful.