If I get my ticket online, can they print it out?

Yes

You can just show it to them from your phone e-tickets do not need to be printed out

Yes and no, depending on what options you make on the sale. Plus, depending on if they have an agent at will-call on the day and time of your trip... But, remember, you can also do eTicket on your phone, and then board, no printing necessary!

They will print it out at the desk, yes.

I selected the E ticket. Greyhound sent the barcode to the gmail account on my phone. The driver scanned the barcode and I boarded the bus; completely by passing the ticket counter and line.

Absolutely yes they can

Lol, based on your personal tastes from past reviews I wouldn't suggest Greyhound for a mode of transportation for you, regardless of the answer to your question ;)