If A or D is not available?
 

I guess this has been discussed before, but I could not find through SERACH.

I plan to purchase AONE5, however, there are no A or even D available on some sectors(regular F or C is available). In that case, I have to sit on Y if only L is available?

Also, if we book L, and then later D or A become available, can we change them without penalty?

Also, since my total sectors become 18, we want to take 2 sectors out from RTW, and purchase roundtrip separately.
For example, NRT - LHR - LIS - LHR - NBO, and we remove LHR-LIS-LHR and purchase this LHR-LIS round trip separately.
At NRT, we have to show two tickets, one for AONE5 and another for LHR-LIS since we want to go LIS through LHR without stopover.
If we use two tickets, and if NRT-LHR delays, there is no guarantee on second ticket LHR-LIS sector?

Thanks in advance.

Yes you would be in economy if only L is available. You can waitlist for the higher booking class in accordance with fare rules, but there is no guarantee it will clear even if the cabin is not full.

If you change LHR from a transit to a stopover be aware that taxes will go up substantially (currently 80 pounds and will be going up further from November). Depending on airline you might or might not be able to through check bags NRT-LHR-LIS across separate tickets, and probably won't be able to check in for LHR-LIS until you reach LHR.

Yes, if you misconnect on separate tickets it is up to the goodwill of the airlines as to how they handle it. In practice as long as you have status and/or high fare you will get rebooked, but depending on availability it might not be same day.

On my AONE4 ticket issued by AA they could put us in a cabin below First on their own flights. There were a couple of BA flights on the itinerary where we were waitlisted for the flight, with plenty of D and L availability. In the end we shifted our flights by a day before booking and were able to confirm A seats on all flights ^