I understand your questions. This issue, as I see it is how Mandrake sees the raid array and how it handles it.
An axample, I had a raid board, but set up as IDE drives. No raid array, just three drives, one connected to the primary raid connector.
Lindows saw the primary drive as hda (as most distros would see it).
Mandrake saw the same drive as hde, in a sense, seeing the primary drive in reverse. It took me a while to figure this out and tell Lindow's Lilo to reverse the drive designations, after finding a solution searching Google.
"1) advice on how I could diagnose the problem "
You already know this, Mandrake is seeing the raid array in a way that will not allow the distro to be installed. It doesn't work right now for your purposes.
"2) obtain advice from the users who have got it running under 9.2 and how they achieved this."
Hopefully someone here may have an answer, but, as you have already done, search for possible solutions and see if any are reasonable for your level of expertise in implementing the solutions.
Usually when Mandrake staff say it isn't supported, what they mean is that there is no easy solution that is feasible for most users, or that is proven to work most of the time (is reliable enough to implement).
The problem here is a failure on install, which often is more problematic than solving an issue once a system is up.
"3) should I be even bothering and should consider another RAID card from another manufacturer?"
This depends on how determined you are to use Mandrake and find/implement a solution. Again, your comfort level in implementing the solution, if found.
I am not a raid expert, but it looks like this will not be very easy to solve. Your best bet is to follow the leads, and if you are daring, try the solutions suggested.
One word of caution - if this is your main box, be aware that you could trash or disable the data already on the drives in the process. So if you are going to experiment with an unproven solution, be careful.
What I would be curious to know, is what happens when you try a cd live based distro? Can it ID your drives... this will give you an idea on how feasible it may be to proceed. Of course, use a recent version of a cd live based distro to give you a proper trial.