If I'd been happy with a computer that long I wouldn't change brands. As I said above.
I don't know the Aladin or what method the used to calculate N2, since I was still using tables as computers were too expensive for my blood. It may have only one compartment for calculation like tables and wash out sooner so it could shut down sooner. The new computers have way more calculating power than the old ones did and may be running more compartments and wait till all compartments clear, therefore running longer.
As for power draw, since it continues to compute until it clears, it wouldn't take much or any more power in or out of the water, thus no change from one to three dives a day.
There is no reason for a new o-ring, I go for years without changing it out. Just make sure it is clean, seated in the groove properly, and the cover is properly set in place and closed. It takes me less time to do it than describe the procedure to you. I use a very little lube on it after a year. If you are not handy, or really worried, take it to an Oceanic dealer or your LDS and explain your issue. I don't know of any shop around here that wouldn't help, 'cause it would take only a minute and be good PR.
Good luck
Bob