3rd rotation starts on 12 July apparently.
There is a big difference between interlining (as per the original press release) and codesharing. Interlining basically allows your bags to be routed through the connecting airport without unloading/rechecking and also protects your connection if the first flight is delayed (ie you get put on the next flight) but prices are normally crazy. A codeshare on MME-LHR would have the baggage & protected connections but would be sold on a BA flight number and you could book this flight on BA.com, get Executive Club Tier Points/Avios and connect to anywhere on the BA network for a similar price to what you would pay ex-NCL.
I wonder if the codeshare delay is down to BA rather than LM or the airport. Timetables are all over the place right now and taking on another codeshare route (a huge pricing exercise anyway) would cause BA more headaches when they move other flights around. Perhaps they are waiting for things to return to some sort of normality?