Hi, to answer your questions:

* how strong are bots usually? 

Depends. If matchmaking "likes" your squad you'll get weak bots. But if it doesn't like your squad bots will be much stronger then you.

* Are different bots different skill levels? 

They have no skill. All their actions are random. But they have diferent equipment. Same are weak but some can be 2 stars higher then you and with legendary stuff.

* How can you tell if you are playing with a bot? 

1) Nick names like you already noticed (DfenestrateU, OrlandoSucks and a fiew more) 

2) Behavior. They act randomly. Walk in one direction and then turn with out a reason or they can just stand still and do nothing.

 * They don't seem super strong either even with that much practice 

They will get stronger and dumber.

Hello! The bots are used due to the lack of online players with approximately the same Squad Power to be paired. The reason for this is that our players are located all around the world and have different time zones. Our team works hard on making bots more close to a human behavior that must make the game experience more fair and interesting/