@eagle61 I think its strange no matter who you are or what region of the world your in ;)
There is no possible way those can not be changed.. If they don't know how to do it, or have no access to the router - I would check if the username/password is just default for the make and model for sure.
Then call the isp for help, those clearly not default.. So even if the isp set them up initially, not like they can not change them.. Its not like they said ok we can set this IP exactly once.. Once you set it your locked to that IP forever! ;)
But no there is going to be no way you can just slide pfsense into your original setup without some down time.. And you sure are not going to be able to route with the same networks on 2 legs of a router..
Lets say you could route even.. If some client on your 192.168.10 network wants to talk to 192.168.10.1 as its gateway.. How would that work.. He says oh need to send this to my gateway 192.168.10.1 - let me arp for that.. ooops no answer, = no access to anything off my network.
So you would have to change the gateway on the client to point to pfsense 192.168.10.x address on the lan side.. So you would have to touch every device on your 10 anyway.. And then still policy route if you wanted specific devices to use a specific gateway.. But you can not do that anyway..
So bite the bullet, schedule some down time with the business and set this up correctly.