IF: How shall an additional stakeholder - e.g. a hospital - who's not hosting the researcher but collaborating with the fellow, be referred to?

They can't be considered as partner organisations since they won't provide additional training nor host the researcher during secondments. They can however be referred to as e.g. "collaborators" or something similar, and their role should be clearly described in the proposal (mainly under the implementation and/or excellence sections).
For example, if hospitals will participate as patients will need to be included in the study carried out, they are not a beneficiary (not hosting the fellow) but they are not partner organisations either (no secondment is needed, nor specific training will be carried out there). So they can be 'collaborators' and the candidate should clearly specify their role in the proposal.