How do I complete Everybody Loves Shen if Shen isn't unlocked until Act II?

Solution 1:

Unlocking Shen permanently unlocks him for all later difficulties and all future characters, so if you decided to try a higher difficulty with your character you can have those conversations with him or you can create a new character on normal and have the conversations with him there. (I have done so myself)

Solution 2:

Once you unlock Shen in Normal difficulty, he will follow you also on the other difficulties. So basically all you have to do is start a new game in Nightmare mode and he'll be there waiting to talk to you.

Solution 3:

Unlocking Shen in Act II on normal difficulty unlocks him for every difficulty and Act (especially Act I) so you can talk to him in Act I after unlocking him in Act II (you have to either redo all quests on normal difficulty or simply get the conversations later on).

The same is true for Haedrig Eamon.