1) You are right, multiple armour types is a wise move especially early lvling - by lvl 12 I still had not spent any points in any of the armour trees because I prioritise getting my main skills, I don't see myself getting many of the armour passives till before lvl 20 at the earliest. As a result, you aren't losing out on bonuses by not wearing a full set of the same armour type.
By wearing the different armours you can level them all up, by end game you will likely be wearing a combo of 5/2 for armour, so it is wise to have all the passives available to pick rather than be stuck lvling at a late stage.
2) Be aware that while class skills scale off magicka, the melee like skills tend to crit based on physical crit % and NOT Magicka crit % (e.g. Templar Biting Jabs) - as a general rule, any skill that seems like a melee attack will be using physical crit despite being Magicka based
3) This is a little more debatable, earlier in Beta it used to be 10 Stamina, 10 Magicka, 20 Health per point spent in the stats. As a result Health gave the best bang for the buck, in addition, enchants did not scale similarly, so you may be choosing between an enchant that gave 100 to one of your stats, this meant Health was even more obvious a choice.
They then changed it to 15 health per point and I believe enchants are scaled similarly - so you could pick a 100 stamina, 100 magicka or 150 health enchant.
Due to this balance, going all health is not so clean cut, between reaching Magicka and Stamina softcaps and the regen softcaps, even with food I believe you would struggle to get it all via gear, so depending on your buil you may want to split your points then use gear enchants to juggle the shortcomings.
Just to make sure you are aware that staff basic attacks and weapon damage are based off stamina, though I think the skills still base of Magicka (may need to double check, as a non staff user I am fuzzy on the details).
You underestimate the power of 1h & shield. Even with no Stamina focus, the first skill when morphed reduces armour and Magicka resist by 40%, if the rest of your abilities are Magicka based then they get significantly boosted in their damage thanks to that debuff - be warned though that the ability also causes a taunt so may be more a solo thing unless you are actually tanking it anyway