How to configure Mac to route external IP requests to different interfaces?

The route add on your laptop was a step in the right direction, but you only get halfway through with it.

Consider this graph:

NETWORK A___
|          |
|          |
|          |
Laptop     Mac__________
           |            |
           |            |
           |            |
        NETWORK B      NETWORK C

You told your laptop that to get to NETWORK B it has to send the packet to Mac.

Now a computer in NETWORK B gets a packet from Laptop (10.0.1.10), but has no way to reply back, because it doesn't know the 10.0.1.10 IP (or the 10.0.1.0/24 network I assume) is reachable through 192.168.1.10 or whatever applies.

You either need to

1. do IP Masquerading (so the terminals in NETWORK C and NETWORK B believe they're talking to a member of their own network), or
2. add a route back to NETWORK A in each of the terminals you need to talk to in NETWORK B and NETWORK C, or
3. add a route to NETWORK A in NETWORK B and NETWORK C default router... probably with routeback enabled.

I'm assuming OSPF or RIP are unavailable for you.