> Thank you, just to be clear: there is no obstacle to a Protestant sponsoring a child's baptism in the Catholic Church as parent or godparent?

Strictly speaking, there is a distinction between "sponsor" and "witness". The sponsor must be a Catholic, the witnesses can be non-Catholic Christians. However, in my personal experience (having had one child baptised Catholic, and the other will be baptised soon), everyone just collectively treats the "sponsor" and "witnesses" as "godparents". The parish just tells people "one godparent must be a Catholic". The sponsor is just the first godparent listed on the baptism certificate.

As far as parents go, technically there is no requirement that either parent be Catholic, or even Christian. A Buddhist couple could ask for their child to be baptised as Catholic. Now, in practice, priests will be extremely hesitant about fulfilling such a request, but strictly speaking it is allowed.

My understanding is there needs to be a reasonable expectation the child would be raised to be Catholic. A typical heuristic is at least one Catholic parent. I guess "be Catholic" in this context means "will go through confirmation" or something to that effect.

Yes. A priest is not supposed to baptise without such a "reasonable expectation". (Although if he does it anyway, he has broken the rules, and could be disciplined for it, but the baptism is nonetheless valid.)

One Catholic parent is the usual heuristic but it isn't actually the rule so isn't absolutely binding. For example, if a non-Catholic is known to regularly attend Catholic services, but for whatever reason is hesitating in formally converting, a priest may very well agree to such a parent's request to baptise their child.