To make a long story short - I had ticketed a flight AUS-IAD-LHR-DUB through United, with LHR-DUB operated by bmi as a UA codeshare. I now need to change the date of the flight to another day. To make the change through UA would cost $500 with the change fee. However, I can tell UA to eliminate the LHR-DUB leg with no difference in fare, paying only the $150 change fee, and then buy a ticket on the same bmi flight through Orbitz for $100. This way I save $250 or so.

My questions:
1. Would I be able to link the UA ticket and the bmi ticket since they are both *A carriers? More importantly, would I be able to check the bags through to DUB?
2. If not, how much time would I need to allow to clear immigration and customs, transfer to the bmi terminal, and check my bags in there? Would two hours be enough?
3. I could also buy a similar ticket LHR-DUB on Aer Lingus. What is the status of their alliance with UA? Could I link a UA ticket and an Aer Lingus ticket and check the bags through?

Thanks in advance for your help.