I think we might be dancing around the real issue - that the problem is whether to take an enchantment NOW, more than which one to take. The opportunity won't go away until we take one. Which to take is secondary to whether to get one right now - to be better prepared for the SDM invasion.
How much does Mary need that, in light of the other potential risks and downsides?
There's two parts to consider for that, too - how likely is it that the enchantment's difference will be necessary or helpful for tomorrow, and how will the long-term consequences pan out?
We haven't looked at the first one.
There are two (and a half) ways the SDM invasion could go awry in a way that matters to Mary - either the SDM residents or Renko get unreasonable in their fighting or demands leading to a need to intervene, or a third party entering the mix and throwing the whole thing into chaos (i.e. Alice).
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I don't think it's entirely likely that Alice will try to attack the mansion, to finish off Renko or otherwise. She discarded the idea of assisting with the attack as soon as Renko said she wasn't planning to loot the place anymore, and seemed more concerned with the idea of blackmail than the opportunity to bring Renko down/rob the mansion. Does she even know whether the attack is still on?
I think Alice was roughly satisfied with threatening Mary and Renko's sight if they spread word of her planned involvement. If anything, I expect we'd have given her the idea of invading on her own by insisting on adding that to the contract. She might be watching like a vulture in case everyone gets completely worn out, but unless the fight goes to a standstill with no real challengers, I'm not sure she'll try to intervene.
... maybe if we had something specifically to counteract Alice...
Even if she does interfere, then her motives are a lot less benign - if we could make her slip up on why she joins in the battle, then having both the SDM and fairy mob turn on her after getting worn out on each other would be absolutely delightful. And she'd wait until that point to attack, wouldn't she? Unless she went straight for Mary for emotional blackmail, which would distract Renko from the fight... but the results of the SDM attack are secondary in the end, and we have one or two tricks up our sleeve. Would Alice be expecting Mary's World to escape a kidnapping?
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I'm not as sure about the chances of Renko or the SDM getting unreasonable. Renko would be pretty easy to reign in (hence only counting as half an issue), but Sakuya said that Renko would be shown as much mercy as she deserved - or something like that. Vaguely threatening, but not completely. And again, Sakuya doesn't sound like a complete tyrant. Temperamental at the moment, scary as hell, not very nice, but not a complete dick, either. We haven't heard anything about Remilia being vengeful; more proud and selfish. I don't know if she'd make a point of really hurting Renko if the invasion failed.
I think this one would hinge on whether they'd feel any sympathy after winning if Maribel made a plea for them to not be complete dicks - if it even comes to that.
Basically, this is only an issue if the invasion fails, Alice doesn't intervene, and the SDM demands unrealistic compensation for it. It's the worst-case scenario, in which case Mary would basically be taking on the entire SDM for being unfair after winning the fight. I'm... not sure even Byakuren-style strength and the potion would be enough for that.
... on the other hand, who says we'd have to make them pay immediately? I expect the SDM would be willing to delay an unreasonable swift and immediate disproportionate punishment (death, blinding, dismemberment, etc) if Mary offered to do something for it. Whether we actually do that or go off to go xyber huge to take them down would depend on how reasonable or noble their request is.
(Tangent: It would only be fair to make sure the SDM residents get enough food to live on even if the pantry gets raided, through our help or otherwise. We have no idea whether that would be an issue, but if it would be...)
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In short - there are relatively few scenarios where the power boost would be directly and significantly relevant for the fight and aftermath. Only, really, if Alice tries to go after Maribel, or the SDM's retribution is disproportionate. In the former case, I think we might be able to take Alice on with just the potion and a little trickery, and/or the help of some of the others involved (which probably wouldn't be too difficult to sway). In the latter... I think we'd be in trouble strength-wise regardless (especially without the help of the items Renko'd use during the fight), and I'm not sure how likely it is to get that bad - staving off Renko's looting would be worth something to them, and maybe staying out of the fight. Not as much helping her get items to use for it, but would they be able to know that? Rinnosuke doesn't ask or answer many questions, after all...
... a lot of this is guesswork, but we need to take into account how important it is to actually do this for tomorrow, regardless of which enchantment is taken. I still favor getting the full-blown one if/when (more likely when) we get one, but I'm not sure it should be done tonight. There are many possibilities that would avoid the need - Renko wins in the first place or the SDM is noble or at least reasonable in victory; Alice diverts the entire fight having to deal with Mary and anyone still standing on either side, or Alice doesn't show up at all. Those that do are of uncertain necessity - taking on the entire SDM after the dust settles might simply be suicidal regardless, and there are ways to circumvent Alice.
Do we need the enchantment tonight?