For the main campaign I'd advise against a full-mage-party. One or even two full casters with buffs, debuffs and crowd control spells will definitely stay useful until the end game but you will need physical damage later on. In fact option 3 is a much stronger end-game team than the others
Elemental combos and terrain interaction is one of the strong and fairly unique suits of the game, which is why I'd recommend to keep at least one full caster. You don't need to limit yourself to two schools, though. Not every skill has to be maxed. In fact, creating a versatile caster with 5 magic disciplines is better than maxing two. Also, you might want to give this character Loremaster 5 to be able to decide quickly which elements to use in a fight.
There are better summons than spiders in the game, yes. However, imho summons are pretty useful at the beginning but tend to be weaker as the game progresses. There are also better substitutes (e.g. charms).
It should be fairly straightforward to progress on hard with a completely non-magical party as well, so I doubt you'd have real difficulties with option 3.
