RE: Arched back with shoulder press
Not sure what u mean with the second sentence mate.
i've also got a 'hollow' back. you need to tense you midsection before and during the lift. don't use too heavy weight at first until you get used to it.
i'd look into doing some core excercise work. the plank and side bridges. I injured my back on boxing day and physio gave me core exercies to do and it help immensly.
don't resort to sitting on a bench to do the exercises as this will exacerbate the problem.
Pain is temporary. Success lasts forever