Shared XP if you're looking for an all-rounded account or looking to train it to maxed otherwise you'll have to use an alternative method like Dharoking to train Attack and Strength.
Guthans also heals and can afk it for a long period.
Definitely shared. Otherwise you'll have to train through an alternative method to get efficient strength and attack exp, would would eliminate the benefit of NMZ essentially, because you'll have to constantly eat food.
Shared, unless you're training with something other than Guthan's and simply using the set to heal up, then you should be focusing on Strength as this boosts your melee xp/hr later on.
I don't think it's worth training with Guthan's anymore, it's really inefficient. There's much better alternatives out there unless you're going to be completely AFK for extended periods of time and have a workaround for the timer. Controlled is (marginally) better DPS and is the better option though.