To answer your questions, 1.)yes/no (it really depends on certain things) 2.) Yes 3.) Depends.
The reason for all the goofy answers are because it is all situational. Since Sprint is a CDMA network if your moms phone is from ATT the answer is no because ATT is a GSM network which takes a SIM card to activate.
For your second question the answer is yes, you can switch back and forth through phones as long as the phones ESN is still good.
Thirdly, your question regarding billing and one time charges can vary as well. If you currently have a smart phone your monthly charges should not change unless you change your plan. If you are using a "dumbphone" you will have to pay a $10th surcharge monthly. Lastly you may or may not incur an activation fee, this I am not entirely sure on though.
My Mom has an iphone from another service provider that she is not using any more. I would like to try it on my Sprint account. Is it possible to activate that phone in the place of my current phone? If so, could I go back to my original phone if I do not like it? Also, would that activation generate any change in my monthly bill or one-time charges?
After damaging my phone I found a used one to replace it and I can't figure out how to make the switch...any ideas?