Hi, regarding your second question: Meraki Hub can properly communicate spokes even when they do not share a common underlay. AutoVPN in hub & spoke model covers that scenario without nothing special to be configured. You only need Hub connectivity to both underlays. Imagine you have dual underlay spoke and one loses underlay1 at the same time the other loses underlay 2. They could still communicate each other.
Regarding your first question: An MX hub in concentrator mode does not allow you to configure static routes. You could theorically run ebgp in the underlay against the PE (asking Meraki to make it available). However, IMHO it would be a case that does not fit with the purpose of positioning a MX. eBGP in Meraki MX is intended for learning local subnets and propagate AutoVPN learnt routes towards your eBGP local peer and still use AutoVPN (and its embbeded ibgp for wan connectivity)
MX hub in route mode can be configured with lan side static routes. However I guess it does not even support eBGP over lan yet.
Regardless of the hub type, uplinks do not support eBGP. Only AutoVPN runs ibgp among meraki MX peers.
Regards.