I believe you can claim the original TP's despite the re-route, there are various threads on here which will advise on how to do that. (Search function on iPhone is useless I find I'm afraid)
In relation to the cost element, not much can be done. BA sold you a trip from DUB > LAX return and they are on course to get you back to Dublin with a 2.5hr delay. The fact that DUB is not your final destination is irrelevant to them I'm afraid and while the extra cost you will incur is annoying, the savings you made from starting in DUB vs. LHR for this trip obviously made the extra journeys to / from DUB worthwhile, so you presumably are still going to come out ahead on the overall cost?
Ex-EU's always carry a risk but the savings usually outweigh that risk, so unfortunately if this delay is too much for you, avoid the hassle and just start / finish in London in future.
Pilot37