As a long time WHM main:

1.) I wouldn't bother with materia or worrying about stats until you are max level.

#2.) Generally speaking, if you wish to raid, usually people will meld Vitality in their accessories while doing prog raids. As for stats to focus on, it can change a lot. I think if I recall correctly, Direct Hit is stronger at the start of an expansion and Critical Hit becomes the end all be all by the end of the expansion due to how much of stats you can stack, etc. I would think WHM would have enough MP economy to not need to stack Piety (hasn't been a thing since 2.X, but we'll see). Last raid tier I melded Crit > DH > SS and it worked well for me.

#3.) Leveling food, I use just anything, I bought a ton of Easter event food two years ago and have been using that because it's cheap EXP. Stats wise on food, unless you're raiding, it's not needed or required for anything else, it's just bonus.

#4.) For raids, DH/Crit/SS food is good generally speaking. But that's for raids, everyday stuff, most people do not use food for, if you do, whatever is cheap and has those stats or is easy to make for you, is good.

#5.) This depends on your play style, in normal play, most people do not use stat boosting potions, but if you do raids, then after progression raiding, when it becomes farm, then people tend to use MIND potions. Or if your group is super close on killing a boss and need just a smidge more DPS, then MIND potions can be helpful then as well. Other than that, I keep MP potions around for normal play because there's always the chance of one of those runs where everything goes wrong and MP is super needed because you spent it all rezzing and rezzing again a bunch of dead people. xD

Bonus Question
WHM has Regen and Medica II for HoTs
AST has Diurnal Sect for it's Aspected Benefic and Helios for it's HoTs.
SCH gets HoTs from it's fairy skills.