Bettering your form is always good. Where the bar ends up touching on body is determined by lots of factors e.g. arch height, grip width, tuck vs flare, limb segment lengths etc. so unfortunately there’s no easy answer.
Experiment: Do some sets with 75-80%1RM and do a couple of reps touching higher or lower until you find wherever feels best, strongest or allows your to move the bar with the most speed.
Biomechanically I’ve no idea. Heard decline bench activates more chest but lowering the bar more towards the stomach does not necessarily achieve this effect. If you arch more and the bar happens to touch below your sternum making it more like a decline press sure there’s more chest activation. If you are forcing the bar lower than maybe more delt action going on.
Either way a bit extra activation means fuck all to the amount of muscle you grow, where you should be focusing on overload instead and strength/the amount of weight you can put up.
Forearm or upper arm? To make internal fixative devices that restrict range of motion is kinda defeating the point. With proper rehab most return to full function ROM, strength and increase these attributes beyond pre injury levels. These peeps cannot use their internal fixation device as an excuse. However your injury may have been particularly severe. What kind of fracture/s did you sustain? What current limitations does the plate cause?
It’d probably be safest to check with your doctor/surgeon or other health professional (decade later is probably too late lel)
Is dis a formcheck… there’s no tag. No formcheck for you