Originally Posted by
blackllotus
Stretch seats will get you more than 4 inches of extra space - 5 to 7 inches according to Frontier's website.
Buying a ticket does not entitle you to a carry-on regardless of whether or not someone is occupying the seat. If you try to bring a carry-on past the gate then you will most likely be stopped and charged. Carry-ons have to be stowed during departure so you wouldn't be able to avoid using the overhead compartment.
As far as I can tell there is no issue with putting personal items in the overhead. While the personal item must fit under the seat you are not required to put it there. On all my recent flights (10 in the last several months) the overhead bins have been far from full, most were virtually empty.