Its a minefield mate as it could be many things you definately seem to know your stuff with the IAW checks.Im only posting suggestions at basic level as you know ,hope someone comes along from a professional view.
The filter is something I,d service regulary , As for me saying injector it could be a case of they need cleaning with bad spray pattern dumping fuel rather than spray. I actually think its something else tbh I would imagine injector issue would be acrosss the rev range not just idle and low loads.

Not sophisticated suggestions but you didnt mention them so I have so readers can see more info from you.

hth
Mike