Preventing Multiple DHCP servers/Vlans

[platypus]

Pity you dont have a dedicated switch for this, get everyone to plug their PC and the routers in for testing, so they separated from rest of LAN, but can still do the remote commands etc.

Indeed.

Not likely
I think your biggest issue is not the DHCP but the rogue routers using the same IP address as your gateway.

It's not, as I said above the gateway used for net traffic is different to the default ip of the routers. It's a different subnet even.

Or get some good switches that allow you to set-up lists of authorised devices.

If only that were an option!