I lost the return envelope that came in my rent bill. Can I use different envelope to mail my money order?

Sure, they just send one because they want to remove any excuse for people not sending it on time.

yes

Sure. Any envelop will do.

Yes, that's how it used to be done in the past, it's still valid.

That's the way I have always paid my rent bill and many other bills. You are never required to use the envelope they gave you. In fact, most of those go right into the fire for heating at my house, since I rarely use them.