Limited response with Expert Mouse (by 2 users)

I have an Expert Mouse 64215 V5.0 Ser No 26960309BD which I am trying to use on a Dell Dimension 2400 via the standard 9-pin serial port. [I use an Expert Mouse 64325 USB on another machine and am trying to replicate the functions for consistency - apart from scrolling, of course].
The latest version of Mouseworks (6.2) does not recognise any Kensington pointing device. However, V5.6 'sees' the 64215 (Serial port 0, IRQ 248, 4 Byte Protocol) but only responds to single button clicks - it does not respond to chording on either pair of buttons, nor can I assign 'Middle-click' within AutoCAD (my main application), though 'Drag' operates as normal for 'All other programs'.
In addition, any custom settings are lost on rebooting the machine.
I have tried attaching the 64215 to the PS/2 mouse port via an adapter but then the device is not even recognised.
What can I do to access the full capabilities of the 64215?

I have sent this query three times to Kensington via their websites - twice to 'europe.support@kensington.com' and once to Kensington US and each time it comes back as 'Delivery failure' with the target 'graviskensington_support@xtrasource.com'
It would seem that Kensington have no interest in supporting their products.