Orange take the HTC and add their own custom Extended ROM with their network lock, colours, backgrounds and programs.
When you hard-reset the device, it sets up Windows Mobile and then runs the Extended ROM automatically.
Therefore the only way to "de-Orange" it is to flash the unit with an Extended ROM from the original HTC version of the device or a custom made one from a developer. The issue is if Orange requested any hardware changes, as if they did then flashing the ROM could brick the device, killing it, often permanently.
If the ROM change is available and worked (a big if), then once Orange's additions are removed, the phone would work on any Network as this should also remove the Network lock, so yes, it would still work with the Orange network.
We do not sell the Orange SPV here at devicewire, we only sell unlocked Network-free phones, so the information on this Orange device is really out of the remit of this forum.
There is a wealth of information on the why's and how's for your Orange SPV over at www.xda-developers.com which is the premier website to visit for this kind of ROM work, information and advice. Registration there is free.
Regards
Ivan
Devicewire.co.uk