Final 3 greens. At major airports, emergencies will be given priority but, inevitably, there will be traffic ahead; you simply cannot close a runway for an indefinite period. ATC will provide more separation to ensure a landing clearance but there is no instruction I was ever aware of which specified what you have described..
I think a good few people on here don't understand that Mayday and PAN are simply communication codes to indicate to ATC that a problem exists. Subsequent action taken by ATC to alert emergency services then depends on what the crew tell them about the problem.
I don't think any pilot would be criticised for calling a Mayday which was later found to be inappropriate. Better to be safe than sorry.