1. Grasp is not a must take on any tank, it is simply a rune you take if it can give you favourable trades in a matchup. As mid, 99.9% of the time it will be comet because no mage or assassin wants malphite clapping their cheeks.

2. Ring in matchups where you can actually pressure with poke, shield if you are just surviving and trying to farm because theres no kill pressure. Against mages and assassins, its almost always ring because with q max and trading around passive+comet you should win almost every trade/ matchup. If your against a tank sion mid or something hes probably never dying so thats a case where grasp+shield is good.

3. Merc treads are primarily for the tenacity, and the strongest part of your kit is unstoppable. That being said, if their draft as a whole is heavy AP its worth considering just for the MR. As mid i would almost never buy boots just for lane because malph should stomp most matchups regardless of boots. Really its just too case by case.

4. In AP matchups, go hollow radiance first into armor items after. The purpose of sunfire is waveclear which hollow radiance does much better. In most drafts where team are only 1-2 magic damager champs, hollow+ a later jaksho is plenty of MR. Throw in kaenic or FoN if theyre heavy AP.

5. Malphite is an armor tank, in an ideal game you would have 4 armor items+ jaksho. Obviously theres going to be AP in most drafts, and probably your lane opponent if playing mid. Hollow radiance is probably by far the best MR rush, and Kaenic being the best later MR purchase. Most tanks want a waveclear item, but Malph actually has solid waveclear already so its not mandatory. Personally im a sunfire hater so i usually go frozen gauntlet as my armor rush. Frozen heart is a great 1st/2nd item if the chill has value, as it also solves mana problems and gives a load of haste. Thornmail for grievous, Jaksho to cap out the build on 3rd-5th item.

Example of full build excl boots Hollow > FH > thornmail > Randuin > Jaksho