After the initial set-up, the central screen should have one big fat Sentinel button to be pressed and everything else miniaturized. This set-up would increase the scope of the pressing area in panic situations. All other things can be shown as small icons.
There is no need for instructions to appear all the time.
In addition to the text messages, Sentinel should be calling the three numbers continuously until at least one number picks up. An SOS feature some phones already have.