PF: Is it just a strong recommendation that proposals should be submitted by a representative of the beneficiary (e.g. supervisor of the proposal) or does the submission system prevent the researcher from submitting the proposal themselves?

On p. 5 of the MSCA PF Guide for Applicants it is stated that indeed the proposal should be submitted by a representative of the beneficiary. This should be read as a strong recommendation. Technically however, the researcher can also submit the proposal in the portal as in previous years.

PF: Who submits the proposal: the host institution, the fellow or the supervisor?

The Funding and Tenders Portal (FTOP) submission system has been set up in a way that MSCA PF proposals can be created and submitted by a designated contact person from the host institution, the supervisor or the researcher. Any person creating an application in the FTOP system can give access rights to other parties, allowing them to submit the proposal ("full access"). However, as the submission of the proposal and other actions that follow this procedure (such as a withdrawal or, if successful, the grant agreement signature) ultimately fall under the responsibility of the host organisation, it is advised that the host institution submits the proposal on behalf of the researcher (EC FAQ 911).