Connecting from J (Biz) to Y (Econ) lounge access
 

I have an inbound flight to MAD on IB in Business+ connecting to 'separately booked' Economy flight on BA to LHR. Will it be easy enough to access the lounge at MAD during my connection using my inbound J boarding pass ; this should be the case as per the One World Lounge access rules. However I know sometimes the reality is different.

The rules you posted specificalyy EXCLUDE access for arrival lounges....as far as I know!

That is talking about access to and using dedicated "arrivals lounges" - which are lounges provided by some airlines specially for arriving passengers. They are not covered by oneworld rules, only the airlines own rules.

pazza2000 is talking about using a normal departure lounge, as a departing passenger, when connecting onto a flight.

As you note the rules are clear.
And that is what the rules are supposed to be in practice too.

I've no direct experience trying it at MAD, based upon inbound flight, personally I would hold your ground if denied. Ask for it to be checked it.

Keep a print out from oneworld.com with you to arm yourself.

Oneworld rules definitely apply at MAD with regards to connecting flights.
Just need to show inbound boarding pass.
As suggested above, keep a print out of the rules just in case.