Some answers: It is AHCI. Tested AHCA->DANI, DANI->AHCI, Compatibiliti mode, etc. Basically everything what come to mine mind.
If it will use AHCI, it should work with AHCI. Since this is a new machine, I expect that all of the devices will be SATA. That should be set to AHCI, and you don't need the Dani drivers. REM them out, in case there is some conflict.
Even the demo5 DVD stops on the same place
That would be expected. The demo uses the same setup, but the real disk (except FAT32) is hidden, somehow.
But I do not still understand
You, and a lot of other people.
Even when HD is touched by WIN, it should continue to boot
One of the main problems with LVM, is that it insists that it must understand everything, or it will do nothing (with no apparent way to override that). That is supposed to prevent damaging something, but in most cases, there really is no problem.
Seems in your case the system can't find and load from \os2\boot consistently
I think that the problem is that Win sets up to use too many sectors (CHS), and eCS can only use half that many (perhaps it is only OS2BOOT that has that problem, since eCS can use those disks for data). the result is that eCS can read the first half of a logical track, but not the second half. It depends, exactly, where the files get put, whether something will work, or not. When the partitioning is done by eCS, or DFSEE, the number of sectors is set to something that eCS can understand, and windows can also use that. The "problem" is that you cannot just change the number of logical sectors, without wiping the whole disk. That gets complicated when there is a recovery partition involved (I do hope that you made the recovery DVDs, so you will be able to get back to factory defaults). The other problem, is that halving the number of sectors, doubles the number of cylinders. If that number becomes larger than what eCS understands, you still have a problem.
At the moment, I suspect that windows has set it up with too many sectors.
http://hobbes.nmsu.edu/download/pub/os2/doc/TAMING_WIN7_V0_2_1.zip describes one possible way to work around the problem (and it is somewhat dated), but I have no idea if it will work, for you.