I think you're right when you say
"This solution means that the guest connectivity goes as follows:
Client > Access Point > Controller > Internet."
but i'm thinking that you can connect the AP directly to internet, connect the MSM with the AP thru internet so the traffic goes as follow
Client > AP > Internet Customer > Internet Central Site > MSM > Internet...
It will double the internet bandwidth at central site but it may be acceptable ( WAN usually is much expensive )
By the way the traffic AP - MSM can be encrypted...
As to use a DHCP relay ( in fact if you have MSM in team you MUST USE IT ) you must:
Create a scope for a different network for the address that you assign to guest. (ALL the guest will use these IP so use a BIG scope with short DHCP lease time
Assign the VSC to the scope ( they call it subnet selection) setting here
an IP address and netmask of the same subnet as the range ( but not in the range ) assignet to wifi client
this IP must be used as default gateway and dns server for wifi guest clients.
Obviously you must configure in the MSM a DNS server that will be used as forwarder of DNS request
In this scenario i configure broadcast filtering in the VSC that i think reduce the traffic a lot.