All the questions you asked are relevant, Just wanted to give you some industry insight. Typically for most retail customers, I do not think it matters whether a firm is clearing or not. However, if you are an algo group, prop, large CTA, then it could make sense to go with a firm that has exchange membership due to the back office staff that is dedicated to deal with such operations.
As far as technology and execution: You need to compare apples to apples. If an FCM carries their technology, then you can compare it to others. However, you will find that "the street" uses CQG. Rithmic, TT, and CTS. These companies have their servers for the most part, so being with one FCM versus another does, in my opinion, will not make a difference in fills and execution.
You might see a difference between one FCM and another in the knowledge of the platforms they carry. Although as an IB we do not have the ability to enable platforms for customers, we know from experience if something does not work where the issue may lay. So one question you can add is: "How well do you know the platforms you offer?" Yes, contingency plans are essential in the vent that your net is down and you can not trade. You should have the details when you establish an account.
I hope this helps you.