What mechanics know is usually limited by what they get to deal with in their area. They might also have policy to not disclose problems. I know the two Triumph dealers in my area have torn down several Rockets over the years.
Far as the question though.
[1] If you're talking about the final drive oil seal problem. Best way to tell is whether or not you've had the final drive recall done. If you know you did then don't worry. If not sure, then contact the dealer that serviced the bike. They should have records on it. If you're talking about the torsional dampener bearing problems, then it is a partial engine teardown. The bearing can be inspected and replaced with engine in frame and opening up the bottom casing. A lot of work though. Doubt it could be done in a weekend.
[2] Complete or partial engine teardown. I'm not sure if the transmission can be inspected/repaired by inframe teardown as above. Way too much work if you don't have a good reason to do it. The bike would likely be apart for a month.
[3] This is probably the only thing your friend will want to do, if even. Repairing the rattling issue only needs the front casing to be removed (called the clutch cover) and the top cam cover. It is still a good bit of work if you replace the cam chain, tensioners, and sprocket and key. I'd say a full day of work if you don't run into problems, probably a spend a weekend on it. Unless your bike does have excessive cam rattle, I wouldn't worry about it though I might still replace that cam sprocket if you have a pre-2006 machine.
In all cases, you best make sure you have all the gaskets and whatnot you need ordered beforehand or the bike will be apart for another one to three weeks just waiting for parts.