Route leaking between route tables and not to use the specific routes from BGP

I need to create a filter-based forwarding (FBF) for some of my LANs to access the Internet. I am running Junos and this seems to be much harder to do than Cisco's PBR. Anyways, I need to create a forwarding instance-type. To get this to work I need to leak the 0.0.0.0/0 routes from the master table to the newly created forwarding table. I know there are several ways to do this, but I would like to know your opinion about using a rib-group or instance import.

I know it is a stupid question, once I created this PBR default-route pointing to another gateway, would the router still route the traffic from my LAN to BGP with more specific route or would it choose the 0.0.0.0/0 PBR ?

The LANs are located several miles away and I don't have access to these buildings yet. Would I be able to test this using telnet, ping, etc within the MX router once the PBR has been applied?

The reason I am doing this is I have no NAT on my BGP router where the LANs connected to and I need Internet access for these LAN a soon as possible. The original design was this router was the primary Internet gateway, but somehow the design changed. The Internet gateway is now a couple hops away within our organization.