No, the FA's are not allowed to help you install the seat; you will have to do it yourself. They also cannot hold the child while you install it, so it is best to have the other adult hold the child for awhile until you get it installed. (If you can preboard with the seat and let the other person come on later and hand over the child, that's best, otherwise, have the other adult go sit down, and retrieve the child before takeoff.)
The seat you use must have a full harness, and it must be FAA approved; look for the approval sticker on the outside of the frame. Also, measure the base width of your carseat; if it is significantly wider than 16 inches, it might be impossible to secure in a coach seat, even if it is FAA approved.
Did you inform the airline that you would be using a carseat in your child's seat? They need to know ahead of time, as there are several places on the plane where you cannot put it, so your seat assignments need to reflect that. (It has to go in the window position, for example, and not in the row in front of or directly behind an exit row.)
Tips on installation: recline the seat a little bit, raise the armrest, then open out the seatbelt to the fullest extension. Thread the seatbelt through the carseat, latch it, then position the seat while holding the tab end of the belt. Next, place your knee in the carseat and bear down on it while pulling up to tighten the belt; the buckle will disappear behind the carseat. Finally, press the recline button and pull the seatback up to full upright. To release the seat when the flight is over, either reach in through the slots and release it, or since your hand probably won't fit, wait until the folks behind you have left, recline the seat, and if you still can't reach, then go around to the row behind you and reach up between the seatback and cushion in order to release it.
As to the stroller, yes, you can gate-check it. Ask for a gate-check tag when you arrive at the gate, and make sure it is marked so that it will be waiting for you on the jetway when you deplane. The tag goes on the handle, somewhere where it cannot slip off. Remove all loose pieces from the stroller (best to put the hood in your checked luggage, actually) and fold and lock it when you leave it at the end of the jetway. It is best to secure it shut with a strap to make sure it does not come unfolded in transit. You will be asked to fold the stroller and put it through the x-ray at security; if it is too large they will hand-inspect it, but it still must be empty for that to be done (nothing in the basket, no toys, etc.) The baby has to walk through the scanner in your arms.
I forgot: the carseat also will be sent through the x-ray if it fits; if not, they will hand-scan it as well. They may try to tell you that you can't bring it onboard if it doesn't fit through the scanner; but this is not true. Just politely explain that the child will be sitting in it for the flight.