We'll probably have a core group of 4 total players, but two players will be learning it first. Is it easier to learn the game if the 2 people just do 1v1 skirmishes, or would it be better for them to do 1v1 mission(s) or 1v3 mission(s) with the 2nd person operating 3 heroes?

I can't speak for others but we first practiced the game mechanics by doing some skirmish, 1 vs 1 or 1 vs 2. It worked out fine as the transistion to campaign mode isn't very big!

The advantage is that you don't have to learn the whole game by playing the campaign, to master it only halfway.
I think you can enjoy the game more if you don't have to lookup all the rules all the time and it also plays a lot faster if you already know most of the rules.

There are parts of rules that apply only for skirmish (command cards, army building), parts that apply only to campaign (class cards, heroes, threat, progression - although you can leave that for later), and some that only apply to the Legends of the Alliance App (rest doesn't recover damage, how to handle the imperial activations). The majority of rules are the same, so it doesn't matter much where you start.

I might go with the Learn to Play tutorial - maybe solo, maybe you use it only to explain the basics, and the Aftermath mission is also pretty basic, so you can play that even if you don't intend to play the full campaign. Or play skirmish with the presets of the Skirmish Guide. Playing the app tutorial is also one way to learn the basics.

Skirmish is a good way to learn the basics even if you're going to be playing campaign.

The advantage of learning the game starting with skirmish is that you don't spoil the entire campaign because of early mistakes applying basic rules

Skirmish also throws less rules at you at the start. Things get more complicated once you add in the extra rules for campaign like endurance, resting, multiple attacks, strain, strain movement, strain recovery, attribute tests, imperial class cards, imperial threat, etc and it can be a lot to take in at the same time you're trying to just learn the base rules.