[TGP] Shelterwing Rider obsolete?

A lot of the answers to these questions are going to depend on the matchup. Against an unknown opponent, the main thing you'll be looking for at mulligan will be having all your colours. The second to that is having something to do on early turns: you probably don't want to keep a hand with no cheap units and no interaction.

With getting outpaced: if the decks that are outpacing you are decks like Stonescar, Skycrag or Praxis Tokens, this build is not optimised for that. You'll want to look at running cards like Hailstorm if you're seeing lots of those decks. More generally: this deck forces you to make a ton of decisions (Merchants, Crests, Strategise, the question you raise about Enforcer) that require you to evaluate the relative quality of your cards and these vary from matchup to matchup. As such, this deck is much easier in ETS than on ladder, where you have to learn how to very quickly get a read on your opponent's deck.

On Enforcer: definitely I think people hold Enforcers too often, a 3/3 flier is very relevant body. Vs Aggro, putting absolutely anything in the way is a good plan as lategame you'll dominate so you just need to survive. In general, the key question is "are there units I need to silence so badly that it is worth giving up 3 damage a turn?". Vs say Time decks, if Enforcer is your only out in hand to Sandstorm Titan then the answer to that might be yes. A hand full of Makto might need to save Enforcer as an out to Maiden vs say FJS. It can be important as a way to pop aegis without spending a card in some matchups. But in other cases you might just need the pressure more than you really need to silence anything, especially if you have other answers to problem cards in hand.