I will try to answer your questions to the best of my knowledge.
- If I got a B1/B2 visa for doing step 2 CS, can I do some observership/externship (USCE) too??
Yes, you can.
- And do I have to pay for some agency to find me an externship, or there's a possibility of getting one by myself without paying??
You can search for one yourself. It is difficult to secure a position. There is usually a waiting period after you send your applications. You may or may not be accepted to a particular program. So, timing your externship with your cs exam may be difficult.
- And can I have an unskilled job in night ($$/hour job) to support myself and collect some money??
B1/B2 visa does not permit any form of employment.
- And is it possible to extend B1/B2 beyond 1 year (my country is 1 year only) to get into matching and complete interviews for example??
B1/B2 usually permits stay of upto 6 months at a time.You can plan your travel so that you are here during the interview season; or, if you have a multiple entry visa for 1 year, you can leave and come back after a couple of months (within the permitted 1 year period).
If your visa is not a multiple entry type, you may have to return to your home country and re- apply for a new visa. Extending your visa once you are in the US is possible, but, it is not looked at favorably by immigration officials. It may negatively affect you during your next visa application.
I hope this helps you. Good Luck!