3. A 64bit version of Illustrator we could trial.
There is no such thing. Maybe in CS6? CS7? Nobody knows...
2. How we night obtain a satisfactory resolution from Adobe.
Unless this is escalated to the CEO or at least some of the development engineers involved, all you will get is dumbfounded looks from all sides involved. Simply assume that most of these support guys know a lot less than you and I about AI or any Adobe program for that matter. they may know their sales stuff and the technical stuff they have bene briefed on, but they usualyl are not active users. That's the long and short of it.
1. A solution to overcome the problems we are experiencing exporting large files as png format.
I don't think there is any. Looking in my crystal ball, "IMER" just stands for a generic uncaught image format error and any answer I or anyone else could give you regarding that is as productive as asking a random stranger in the shopping mall. Yes, it sucks that AI can't do such a simple thing, but the way I see it, this is a "hard" bug that can only be resolved by fixing the underlying code and that won't happen before CS6.
You almost the solution and this should work well but there are both advantages and disadvantages to this method.
Open the file as a Smart Object in Photoshhop and save it as a .psd then just double click the Smart Object in the layers panel to open the smart object in AI hide or show what ever layers or bjects you want do a simple save clcik go back to the Photoshop document it will still be opened and it will update and now just export it as a .png.
This shoud work for you yu have to adjust your workflow a little but it should be helpful. You should get the same quality output as well.
One thing about this method to be cognizant of is that when you save any changes they are being saved to the Smart Object and not the original file. In order to save it to the orignal file and the Smart Object you have to first do a Save and then do a Save As. You wil then have to open the smart object again the next time you want to do an edit because you did the Save As.
I just exported a .png based on the dimensions you mentioned above. It worked (AI CS5, Mac).
Can you please specify the exact settings you're using when exporting your .png files?