For silly levels of Winner grinds I'd think Yuugi would be the tank, because she's got meiling-grade HP and DEF, and mind is pretty much completely replacable by affinities at that point. In other words, Yuugi with a full ATK build using nukes. Although, you're also the one with 70 kills @.@ I wanted to get many kills, but then the fight was just such a totally boring slog until the final phase that I seriously couldn't be bothered. I'd also have expected Ran in there, because at that point she can afford to use her def/mnd buff every turn and keep your buffs consistently at -very- high levels so you take rather low damage, but at that many kills, maybe surviving isn't the deal so much as simply finishing the slog of a fight. >_> Rinnosuke is kinda underwhelming in LoT1, too. For example Alice is almost flat-out superior, having used them both up through Winner in my last playthrough- Rinno had a little more DEF, but otherwise Alice beat him out across the board and did more damage. Rinno's damage in a full atk build was really sorta disappointing.
Yuugi is in fact one of the members of my squad, but I use it as a nuke rather than a tank, because for the tank slot you need something faster and with less delay, otherwise you risk getting blindsided by Longsword Ringil (which at my levels does ridiculous damage at absurdly low delay). As for Rinnosuke, it's nowhere as bad as it sounds, for a variety of reasons. First of all, Shining Stars of Traumerei does respectable dmg for its delay (for comparison, with full buffs, my Rinnosuke does around 18 million dmg to Winner with SSoT, while Nitori does roughly 26 million with his Megawatt Gun), and World-Shaking Military Rule is very important for Winner phase 1. Finally, Military Rule also makes farming Serpent of Chaos (the main means of grinding at this level) much faster - in fact, as I show below, one of my team slots is basically devoted to Serpent farming mostly.
I'm really curious about your levels/a stat screenshot after the point of 70 kills, though >.> Trance did 100 but didn't have his save anymore so we couldn't see!
There you go!
(Levels are around 1200-1400, while skill levels are mostly around 1000 except for SP which is useless. Affinities are at 500 with everybody)
Quick overview of my team choices, for those wondering:
Reimu: Best healer
Minoriko and Rumia: Second and third healer (I chose them over Eirin and Sanae because their heals are faster, and they also can deal more damage - sometimes, even healers need to go on the offense vs WINNER)
Rinnosuke: Primary tank - I use him to buff the frontline at the beginning and try to punch through phase 1 as fast as possible, because phase 1 is arguably the most dangerous at this point for the high risk of multiple Longswords Ringil (which at this lvl deals approx 1.5 million to the leftmost member)
Keine: Secondary tank - I sometimes bring her on the front to refresh buffs, but generally she stays in the back until Rinnosuke dies
Flandre: Serpent of Chaos farmer (2 Lavatein at full buffs and it's down), and decent emergency nuke with Starbow Break. With Reimu constantly healing, and phase 2-3 being a bit less heavy on AoE dmg, it's definitely sustainable.
Shikieiki: obligatory DEF ignoring nuke for when WINNER uses Ball of Invincibility
Nitori, Suwako, Yuyuko, Youmu, Yuugi: nukes, in order of dmg potential (in theory I calculated that Kaguya actually does more dmg than Yuugi, but she's WAY too frail for this fight - even Suwako is often hard to keep alive for long)
Quick strategy: I open with Rinnosuke/Youmu/Nitori/Yuyuko, World-Shaking and start blasting away. As soon as somebody falls to less than 50% (generally after the first couple turns), I switch in Reimu and begin healing. Hopefully nobody dies before phase 2.
As phase 2 begins, I start bringing in periodically Keine to rebuff. Replacement order in case somebody dies is:
Rinnosuke -> Keine -> Yuugi
Youmu -> Yuugi
Nitori/Yuyuko -> Suwako -> Flandre -> Shikieiki (kept last because if she dies and WINNER uses Ball, it's generally over. Also, sometimes I replace Nitori with Suwako in phase 2 because Suwako becomes more useless in phase 3 with true resistance)
Reimu -> Minoriko -> Rumia (Rumia is sort of a last ditch healer, Demarcation heals for like, 300-400k max without buffs, but Moonlight Ray does decent dmg and Dark Side of the Moon ignores defenses which is useful)
I think endgame viability is definitely far more flexible in this game. You say you think Momiji cannot compete, but she's one of the bulkiest characters in the game, boasts solid attack stats as well and several very nice passives, good base moves and useful corridor awakening (24% atk buff each time she attacks? she hardly even needs to get buffs refreshed!), and her corridor family skill involves three other very useful characters. A lot of them depend more on your party setup than whether or not they're actually a good character, since the more underwhelming ones generally have a good niche or powerful family skill.
But even so, there will be the end parties with the "better setups". You definitely have sweet levels of flexibility with who you want as your bulky attackers, if nothing else, though. There's much better flexibility when it comes to your support characters, but there's certainly going to be ones that are just a little bit better than the other options... for most parties. For example, Rinno is kinda outclassed by Keine after awakening, and his minor edges over her aren't important in the average endgame party I -think-, but if you're running Shou you may want to keep using Rinnosuke instead of her. And even Sanae can realistically play like Byakuren after awakening, and even have some advantages (albiet with some downsides, too).
Yeah it's very hard to build a LoT2 Plus Disk squad without extensive testing, so I'm mostly trying at random^^ So far, I'm considering the following squad for plus disk
Renko / Tokiko / Yuuka / Maribel
Sanae / Reimu / Suwako / Kanako
Kasen / Youmu / Shikieiki / Yuyuko
Still nowhere as sure of its effectiveness, it's mostly made of fan favourites of mine :-P Thankfully, tomes of reincarnation and xp boosting items allow me to "reset" and try other chars without any risk of new chars falling behind and being unable to catch back up (unless there's some weird overflowing bug I'm unaware of)
Anyway, speaking of overflow, I can assure you that I still have not seen any overflowing yet in LoT1. Fully buffed Lavatein does 35-40 million dmg, and Medicine of Life heals WINNER for over 200 million dmg, but there are still no overflows.
P.S.: The "hour count" on the save above is a bit lower than you might expect because, after realizing that some of the chars I were using were suboptimal (Yukari, Kaguya, Kanako) while others were very needed (Flandre, Rumia, Rinnosuke), I restarted the game and breezed through it with 3X Machine God Lucifer on everybody and roughly 300-600 skill levels already. Here is the old save pic before "reset" for those wondering:
(Lowish stats and SP especially are because I've unequipped all the Machine God Lucifer to carry them to the next save)