Arent their abilities kind of overlapping at this point? I mean, what happens if Kaguya, Sanae, Tenshi and Reimilia are on one table?

We haven't really seen Remilia in action yet so no clue on what she does but the other three are actually fairly different. Sanae's ability gives her a tenhou/chiihou whenever she starts losing, which we already saw Youmu get around by sending her into the negatives in one shot.

Tenshi's ability is actually a bit trickier, since it isn't that she's getting tenhou because of the blessing of a god or something like that. If we take what she said at face value she is stacking the wall and rigging the dice throw not through classic cheating like Marisa but through supernatural means. Essentially it's Marisa's strategy except she can't be directly stopped, can alter all walls instead of just the one in front of her, and on top of that she probably doesn't have the limiter of "needing to be losing". If I had to guess the way around it is for Marisa to mess with the wall after it's already been stacked by Tenshi, but that'll be impossible without Sekibanki's cooperation and even then a couple things would have to line up (such as Tenshi going for a double riichi instead of just getting tenhou again, which I think we can assume she did out of hubris?). Either way we'll probably find out more in the next chapter.

As for what'd happen if those 4 played in the same table, unless Remilia's ability overruled it, I would say Kaguya wins. As long as her ability is in effect there's no escaping a tie. Marisa had to basically hold her hostage by breaking the rules in order to force her to disable it, so unless the other three tried the same the princess would have the win on the bag. Or not. Maybe they'd just keep playing forever.

I can't believe I reached this far in this manga even though I don't even understand a thing about mahjong.

literally me LOL

The cliffhanger would be more cliffhangy if I understood what that pon at the end implies!

Akyuu discarded the 4 man anticipating Youmu's 1-4 wait, but since she called pon on the west wind now she's waiting for the 3 man, which is still in Akyuu's hand. Every single tile in her hand gives somebody else the win, so unless she draws a safe tile she HAS to deal into somebody's hand.

If I had to guess she'll (begrudgingly) deal into Youmu's hand, since while that'll give Youmu the win it'll also keep Akyuu in 2nd. Technically she could deal into Doremy's or Shou's hand and still get 2nd place, but that'd eliminate Youmu which would end the plot prematurely.