Can one node run stores on different platforms

Yes.

The same node can run stores on different marketplaces at once. One Amazon store, one TikTok Shop, and one Walmart store on that node is fine.

Merca already enforces this. How login protection works: on one IP, each platform allows only one login. What gets blocked is a second login on the same platform, not a store on a different platform.

If a main account and a platform sub-account should share the node, Allow the sub-account in that node's Account records. See How to add a platform sub-account.