At that height and weight there is a good chance that the Monterey may be a bit overwhelming for her. I would look at the Recaro Vivo which is likely to fit her better (or the Graco Turbobooster).
Boosters can be use in recline positions, although it's not like with a car seat...a sturdy, locked in position. The back can just be adjusted slightly to align with the recline in the actual vehicle seat.
To know how much it's ok to recline the vehicle seat, consult the vehicle manual. Usually there will be language that says "Fully upright" or "Two clicks back" etc. Then set the booster to fit the vehicle seat.