Yes, the intermediate for an RSA cert is currently R10 or R11
There is no difference in the "chain path" apart from the different intermediate. Can you explain in more detail what you mean by that?
See: Chains of Trust - Let's Encrypt
You should know that there are also R12-R14 currently available as backups which could be used any time although hopefully not. And, R10/R11 will more shortly retire in favor of new ones. The correct approach is, and always has been, to use the intermediate chain supplied by Let's Encrypt when it issued your cert. That is, you should not be "pinning" an intermediate.
Note also that for ECDSA certs these have their own set of intermediates
The background for this change is here: New Intermediate Certificates - Let's Encrypt
The API section is good to subscribe to for important notices like this one: Deploying Let's Encrypt's New Issuance Chains
And, no, it is not possible to request a specific intermediate when requesting the cert. This is intentional to ensure that ACME Clients can automatically handle different intermediates being used.