#6. HDG 125 you will be blind to the runway. Are you suggesting starting a descent BEFORE entering the +/- 30 degree cone of the centre line or good visual with the threshold? You don't mention a ROD, but it will be more prudent to wait until good visual with the threshold and the intervening terrain before descending below MDA. Circling profiles are normally designed to allow a normal ROD to be selected when intercepting visual finals.
#1. Agree with wiggy that MDA should be elected in mCP and thus ALT HLD will be automatic.
Regarding the SI locator and 125 radial. If you are flying FMC MAP and HDG SEL then one might be tempted to nudge it left and sniff early to prevent overshoot. If you are flying NDB raw data, below MSA, then one can argue you should wait for the needle to fall 90 degrees and then turn back a little more to intercept 125 radial. Turning early on a twitchy needle has caused tears before.