You can find a lot of this information by searching Meta:
The email sending/receiving capabilities for Discourse are a bit more extreme than Gmail supports, and I believe it’s against Gmail’s Terms of Service. There’s a guide to help you set this up with another provider. Troubleshooting email on a new Discourse install
You assign trust levels under Admin > Users, you click on a user and then edit their trust level setting.
Title lengths are set at 15 characters by default to encourage users to write useful and descriptive titles. You can change this setting under Admin > Settings (the setting is called “min topic title length”).
out of curiosity - what do you find so restrictive about it - all you need is an smtp provider? You just have to be careful in which one you choose as they all have their own terms of service you need to abide with.