As Meta Knight, you need to switch much between styles. The sh fair or retreating rar fair spacing is a great anti-approach and definitely one of MK's main tools. Even more important is to be able to switch fast between your styles. You should have a certain idea how to play the neutral in that specific matchup, for MK it differs more than for Jiggs where you play the neutral fairly the same most of the times. Then you have your combo-stance where you should just maximize followups. Normally, if you can't get followups anymore, you can still get techchases or even smaller positional advantages.
The main reason you can't play MK just in a Jiggs style is he hasn't that much shield damage on his aerials and he hasn't the range of Jiggs' bair. Against shield you need to rely on a mix of dashdance->grab and aerial maneouvering over your opponent, dairing if they drop the shield and just continue jumping and waiting their shield out otherwise. MK's aerials aren't really good on shield too. Summed up, MK's SHFFLing is good in combos/advantageous situations/anti-approaching, but for approaches you need to do something else (note that perfect bair on shield is +1 so that would be legit, same goes for perfect fair with -1 and shuttle loop is then quicker than shield grab).
Shuttle loop is sometimes needed to not get shieldgrabbed (so fast), and of course for kills it is great. Also note the hitbox stretches a bit so you might combo into shuttle loop when you couldn't combo into other moves. If you can choose between a e.g. nair or shuttle loop punish though, I would always prefer the nair. Also be aware on fastfallers at high percents (like 100% against Fox, against floaties the percentage where it combos will be too low to kill for the loop), up-throw -> shuttle loop is a kill combo.
The back-throw doesn't even grant more positional advantage than the up-throw, only b-throw if you expect DI away only then you get combos. Edgeguarding only works consistent against a few bad recovery characters with back-throw, reason obviously being that MK's back-throw has a much higher angle than MK's back throw. The best situation to use back-throw is when you mix up between down- and back-throw because they will DI away against down-throw, turnaround full jump fair is probably best followup then. Up-throwing is nice, it secures you one up-air and one more aerial, preferably a nair, at the right percentage.
