Hello Paolo,
1) load balancing is actually done by CEF on the forwarding level.
IGP have usually maximum-paths 4 so they can load balance up to 4 equal costs path by default.
BGP can load balance but by default it has maximum-paths 1
two options are possible:
using a loopback to perform load balacing by recursion (the BGP session is only one but BGP next-hop is reachable through two or more links)
or
inside router bgp you increase maximum-paths with an explicit command
It depends from your scenario
2)
if BGP is enabled in all routers you can choice to send a default route in BGP but if it isn't you need to propagate a default route in your IGP
Hope to help
Giuseppe
