Moving forward, can anyone with the expertise comment on what is the right way to evaluate an exchange? why was Think or Swim and ETrade okay and insulated from the issues these other exchanges had?

These are weird times. It depends what you want. I think for most people being able to buy GME yesterday is not an important factor in choosing a broker. If you just want to buy some stocks as an investment any modern broker is probably fine. I dont like RH because it gamifies something that should be serious but for most people it works great and low cost is very nice. For me I 'd rate brokers on 1) Security, 2) Cost 3) availability of products like foreign stocks 4) gui usability 5) reliability 6) tax statements 7) margin costs 8) other products like debit cards/crypto. This list should be different for different people.

What can we really learn from orders that never get close to executing?

Relevant to yesterday’s Robinhood actions in $GME? That they were not being blocked from entry at ETrade, contrary to what RH and WeBull were doing and contrary to some reports lumping ETrade into “the conspiracy”.

I wasn’t in GME, but I want to know how my broker is handling unusual market conditions.

Especially in "order flow" schemes, brokers accepting trades but either not executing them or executing them at different prices is a commonplace. It's great that your experience at ETrade has not included this, but if you ever decide to actually trade a "crazy" stock the outcome might be different.