I guess I'll add that unless the character was already built as an attacker, they're not going to suddenly become an attacker now. Characters like Suwako and Eiki for example, who could reasonably switch between ATK or MAG would be something I'd consider, but Sanae's staying in a tanky supporting role, and likewise with Aya (although she's max SPD instead of bulk. She's still there expressly for Divine Grandson's Advent and randoms).
I've never had problems with switching with this set up. I've already had the practice playing through LoT1 (granted, without beating SoC, Twins, or WINNER) without all these fancy skills.
Regarding Marisa, she's just too much of a dead weight if Master Spark nuking isn't feasible. I might have forgotten stuff that she gets from Plus Disk that could change her singular element coverage / deal with that limitation but MAlice Cannon isn't worth playing with an 11 character party otherwise. It also doesn't help that I've played that exact way with Marisa in LoT1 a bunch and she hasn't changed much at all nor got interesting skills to mix things up.
Hm, well, if you don't want to use a character for a nonstandard role, then I guess that's that. It seems wasteful when they're actually good at it, but OK.
For the switching thing, I'll give an example. At LoT2's current endgame (around 540 or so), character SPD generally ranges at about 700-900 ATB per tick normally, with max SPD characters at about 1200-1300 (this is without SPD buffs). If you were to switch a character out that just used a heavy delay attack, your best option would be Aya, who hits that 1300 ATB range (IIRC, max SPD buff is about 1900 per tick, 2500 is still very distant). Aya would consume two ticks before she can switch your attacker back in. Besides the lost ticks every time you do this, during the time she's in, your buffing characters won't be able to buff your attacking character, so if they get a turn, it's wasted. And if you're rotating attackers, then it's terribly inefficient for buffing since you have to buff at least two characters from scratch, which slows down your damage a lot. So the most effective solution is to use an Instant Attack character so you can immediately switch back to your primary attacker, so that you can buff them more effectively since they're out more often.
This works more effectively for bulky attackers that can come into the frontline without fear of a boss' attacks compared to fragile characters though. For those, a character with Effective Formation Change would work out better, since then your attacker would get their turn after a single tick, allowing you to get their attack in and switch out quickly. Otherwise, you'd generally look at 3-4 ticks before your attacker can act. Bosses move pretty quickly (Serpent of Chaos gets 1300 ATB per tick, so that would be 3900-5200 ATB), so it's dangerous to switch characters like this, since even if you get your attacker's attack before the boss acts, you have to have another character switch them out before the boss acts. It also makes buffing them a lot harder since you have less time to work with before you need to switch the character out.
Basically, it's just about managing ATB more effectively. While these skills aren't necessary to use (or even the only things you can use, a heavy focus on SPD buffs and debuffs would achieve similar results, and you can use the two together if you wanted), you can probably get better results by using them. Though tbh (and this applies to the Marisa thing too), with my perspective of the game being skewed toward "how can I break this game the most" (with the additional condition of "with Tenshi"), I'm looking primarily at finding ways to trivialize the game the most, so idk if that kind of thing is what you'd want to go for in the first place. Marisa taking up a dead slot in the party for the sole purpose of giving Alice 30% more MAG does actually seem fairly reasonable to me because you generally don't really need more than 2-3 dedicated attacking characters (although I may be biased with my approach on things here). But even so, Marisa is one of the strongest MYS attackers even without wiping a boss with a single Master Spark since she has the +30% MYS damage boosting skill, gets +15% more damage from her Awakening (I find that most boss fights don't even last 10 turns, so Hakkero Charge Mode is generally worse than Hakkero Custom Mode), and gets the very handy ability to restore 7 MP with every Magic Missile, which, combined with Sanae's Awakening (+2 MP per turn minimum), would allow her to fire off a minimum powered Master Spark every 4 or so turns, while doing good damage with Magic Missile (Magic Missile does about 40% of a 27 MP Master Spark's damage by my estimates). This should be a more effective method of using her than going solely for a fully powered Master Spark, only thing is that most bosses aren't weak to MYS in the first place. Though, even without weakness, it might be a pretty good strategy to use.
Though all of this is just assuming you'd rather keep Alice than replace her, and tbh I'm not sure how strong she is relative to the whole game. Just pretty sure she would be one of the strongest attackers in the party with MAlice Cannon active.
I still don't know if I want to include Marisa in my final lineup, though that has more to do with me being (1) disappointed more in Alice, rather than Marisa, and (2) more interested in other excellent AoE options (unlike in LoT1, there're a plethora of good AoE nukes to choose from)
Just curious, what are you currently planning with your party?
Marisa can choose a sub like ArchMage to get some valuable elemental coverage, once you hit postgame. Kinda pricy, but she's got MP. Delay would be sort of painful as far as bosses go, though.
tbh the damage output on Archmage spells for their MP cost is so bad that Marisa would run out of MP against most bosses even when striking weakness. She only gets like, 5-6 uses out of them by the time you get Archmage, or something like that. A Grand Incantation character (like Reimu) would probably be able to make decent use out of them, but otherwise Archmage subclass spells are too expensive for the damage that they deal, and for boss fights, it's normal to run out of MP. Southern Cross in particular is very weak and wouldn't even outdamage Magic Missile (which costs 2 MP as opposed to 10) on a 60 CLD target if it was neutral to MYS, with Sheer Force being enough to generally ensure that even MYS resistant targets take more damage from Magic Missile. Post-Awakening, Hakkero Custom Mode only boosts Marisa's personal spell cards, so she has more incentive to use her personal spell cards instead of subclass spells.
Execution is kinda somewhat usable but realistically, most characters will still run out of MP before they can beat the boss with it. Archmage is honestly kinda bad as a subclass.