So can we say that any packet going out of f0/0 of R1 will have source IP 192.168.13.1 (i.e. IP address of tunnel for which f0/0 is configured as source)?
The answer to this is No. A simple counter-example would be any packet going from HQ to the ISP. Presumably, the ISP has no idea that the Tunnel IP space exists, so the packet must be sourced from 192.168.12.1. The key for HQ figuring out whether to use the 192.168.12.1 or 192.168.13.1 address would be what its routing table says is the next hop of the destination. If the next hop is in the Tunnel address space, it will use that.
I think we cannot configure one interface as a source for more than one tunnel…right?
You are correct. While you can have a single interface be the source of a tunnel with multiple destinations (DMVPN does this), technically, this is just a single tunnel. In order to have multiple tunnels, you will need to create loopbacks and source the tunnels from there.