Thunder Clap generates only marginally more threat than Battle Shout or Demoralizing Shout, while your Defensive Stance gives you 130% base threat (untalented).
If you are untalented into Tactical Mastery (which would allow you to retain up to 25 rage, if talented 5/5 when changing stances), then the decision may be more trivial for you.
If you have no rage, but would like to start with Thunder Clap, you could consider starting a pull with Charge (generates 15 rage), Bloodrage (10 rage) then Thunder Clap, dump whatever rage you have left, and swap to Defensive stance, then do your thing. See if that is helpful for your play style.
I ran SM mostly in melee cleaves, and other than the first few pulls at the end of Cath (in the actual Cathedral (and the very first pull, as you enter the courtyard), I didn’t LoS at all. The main reason to LoS is to give some time to pick up runners.
Maybe. You’re entitled to tank however you prefer, and if you prefer tanking in defensive stance with a shield and sword, because that’s more comfortable for you, or suits your play style better than using a 2 hander and stance dancing when needed, then that’s your choice.
If you don’t have macros set up for swapping weapons, changing stances, etc. but would like to, have a look at my tanking guide for a starting point.