When I play jungler I try to focus on 2 lanes. Top/Mid or Bot/Mid. Trying to cover all 3 lanes is much more difficult especially without mobility boots. This works well when I am duo queued because my partner is usually in the top lane and I trust him to win regardless.

If you are winning your lane, and their jungler (and Shen in this case) is unable to slow you down significantly I would view that as an opportunity to focus on other lanes.

As an example, if you are on purple side and winning in bot lane I would try to take control of their Blue Buff and give it to our mid. You are pulling lots of hate down on bot lane, so I am going to punish their top and mid by taking buffs/towers.

It really depends. Yes, he should gank your lane to help you snowball, but no he shouldn't waste time if you guys are doing well. This is very situational. It depends on a lot of things like match-ups, objectives, and etc. I'll try to provide a guideline what I mean
A) If the champ being fed is a hypercarry you have to pressure him. Ex. Nasus, Veigar, Darius, etc.
B) If your ad carry is a strong late-game monster you should feed. Ex. Tristy, Vayne, etc.
C) If your jungler is good at turret dives. Ex. Lee Sin, Malph, Naut, Mao.
D) If your jungler is good/better at counterjungling/holding lanes. Ex. Mundo, Amumu, Shen, etc.

It really comes down to the jungler's preference and their skill level and champ. I know on a tank, I'd try to feed bottom, but on a bruiser or high dmg jungle like Jax, Trynd, Olaf, etc. I'd be trying to pressure the snowballing lane (if it's not a counterpick) because I can safely with those champions and go blow for blow (usually). Tbh, it all falls down to the skill and champion of the jungler. Both ways are good, both ways can be effective, but it just depends on the game. I hope that helps, but that's the best answer to your question.