1. Explorer mode are for people who don't want to be challenged by gameplay. If you want to play the game then I'd say Classic is fine, but if you're really just in it for the story and don't want to meet a lose condition unless you genuinely messed up then Explorer is your pick. Personally I play for challenge, and find Tactician in the mid-late game rather simple. I don't think you'd find the game hard on Classic but if you aren't interested in the game, and rather want the story, then don't feel pressured. There are no in-game consequences for picking between these three difficulties other than the challenge the enemy stats present.
That said I would recommend playing it at its recommended difficulty leave, Classic, unless you are genuinely concerned about your competence at playing videogames. There is a LOT of fighting to do and trivialising all of it probably wouldn't motivate you to finish the game.
2. Those people are very strange and I wonder why you get your guides from very silly people. Playing a double lone-wolf is definitely the easier option, but the game is easily winnable on all three difficulties with any combination of characters. 4-player parties are not meaningfully weaker. On my second playthrough with 4 characters for the second time, I have encountered no issues.
3. In Tactician having a 3\1 balance on damage types is good, ideally you want a character who can shift between either; like the Summoner. On difficulties below Tactician, so Classic and Explorer, these considerations are not important. You can complete a Tactician game with no real difficulty with a 2\2 set-up.
4. My advice is playing the game and not obsess over min-maxing, min-maxing has no place outside of Tactician. Launch the game, pick your character, and see what happens. You get free respecs at the start of Act 2.
5. Custom characters don't have unique dialogue with some characters, but you still get to explore 99% of the game anyway. I don't believe there are any character-locked regions. As Custom characters have some unique settings for themselves (though less), I would honestly suggest playing a self-insert custom character, and then recruiting 3 origin characters (presuming you're playing singleplayer). Some of the origin characters can be a little overwhelming personality-wise and you don't really explore them well when you're playing them.