It is easy enough to get to Oslo sentralstasjon from Bondeheimen, it's just that with luggage, not knowing the city, the walk from the station is something that I would avoid. Personally, knowing that part of Oslo and Oslo public transport pretty well, I would take a local train to Oslo S, and then a tram to the same stop. But the local train is infrequent, so the saving (about half the price) is probably not worth the hassle.

As for the replacement buses for the Bergen line trains, they actually leave between 10 and 20 minutes later than the train that they are replacing!

According to www.nsb.no the bus for the morning train to Bergen leaves at 06.49, and they leave from Oslo S. There is a dedicated bus-for-train (Buss for tog) platform at Oslo S, on the seaside (opera house) part of the station. When you enter the station building from Karl Johan it will be along the tracks, at the far right side of the station