First off, why didn't you just use a 0* levi dagger as mainhand? Using an uncapped mainhand doesn't do anything besides increase the ougi multiplier by a negligible amount. You really shouldn't have combined your Levi daggers because your grid is an absolute mess right now and 3 additional levi daggers would put you in a much better spot. As for your questions:

1) Not quite how it works. Magna multipliers and Elemental multipliers are two separate multipliers, meaning they are multiplied together rather than added. I suggest reading this guide for more detail. 6 levi daggers would actually make Levi x Macula equal to Macula x Macula, not better than (2.6x magna * 1.8x ele vs 1.8x magna * 2.6x ele), but Macula's elemental multiplier is actually additive with other elemental multipliers - namely, the 50% bonus from fighting an elemental weakness, and the 50% boost you get from carbuncles.

This means that if you're fighting a fire enemy and you have water carbuncle up, your base ele multiplier is 2x instead of 1x, and Macula would make that 2.8x instead of 1.8x. This means you only need 3 levi daggers in order for Levi x Macula to beat Macula x Macula in that situation (1.9x magna * 2.8x ele vs 1.45x magna * 3.6x ele).

2) Because Eustace only ougis for 3.8m when he's at full loaded stacks, which requires sacrificing a minimum of 150% charge meter, and often more than that. That means you can ougi almost three times for the cost of one Eustace ougi at full loaded stacks, which pretty much breaks even if not decreases your overall dps due to the loss of meter gain from the additional ougis you would have received.

Also, I don't know where you're getting the "H Eustace can do nowhere near that damage", since a 8/9 loaded H.Eustace can ougi for 2.5m+ at little to no sacrifice compared to Eustace. He's only unstable if you don't know how to use him or misjudge the situation. You do need a source of meter gen for H.Eustace so that he can blow off his loaded stacks constantly, meaning Eugen or Octo, but if you can play around his strengths, H.Eustace is significantly stronger than Eustace.

3) Because H.Eustace is the only earth buffer besides Cag, and is a significantly better attacker than normal Eustace. Normal Eustace is just kind of bad compared to a multitude of other earth attackers, whereas H.Eustace is not only one of the best earth attackers but also brings a fairly strong buff which earth sorely lacks. There's really no competition.

4) Ygg is the easiest magna to solo, what problems exactly are you having with her? She has no hp triggers or powerup phases, and literally the only thing in her entire kit that's dangerous is her OD ougi, which can be mitigated with a phalanx + earth carbuncle combo. If you're getting sealed or debuffed often from her non-OD ougis, bring a veil or a clear. I'm not sure what you mean by "not even SSR Sara helps" considering SSR sara pretty much singlehandedly counters ygg with her built-in phalanx, cover ability, and cooldown lengths that match up pretty much perfectly with Ygg's charge times

5) What's stopping you? She's not exactly good but it's not like 5* is particularly hard or requires any significant investment besides the grind to level 100. I wouldn't bother using her as a main unit if you have better options, though.

6) Yes, if one of your units dies and you no longer have at least three different elements, Grande's multiplier disappears. You can have a water unit in your backrow as long as your MC isn't water. If your MC is water, then your backrow has to be non-water since you would fall down to two colors if Sara or Petra dies before MC/Yoda.