There shouldn't be any sighting of a completed Space Battleship Andromeda in this series. (though a plan for it maybe) It was launched in story for the second film/series. So it won't be ready the first time Yamato comes home.
I hope to see Kirishima again though. I half expect it to fire a nice parting shot to a Gamilas warship with its bow cannon.
As for Niimi. I too think there is a connection between Kodai Mamoru, Sanada, and her. They are all around the same age (late 20s). A friend of mine speculated that if things had gone another way, she'd be Susumu's sister-in-law.
From what little we know of Chapter 5 at this time, I suspect Episode 17 will be the Yamato 2199 version of the Space Fortress episode, where we got most of Sanada's backstory in the original. Though I don't know what will happen to Niimi and the others after the mutiny (Shima will live, but he could be in the "dog house" with the Captain and crew for this one).
Your right about the Andromeda not showing up (of course, we do not know what the script writers have in mind). Right now I believe that there are ships under construction which we may or may not see.
I agree about Niimi (sorry for spelling her name wrong in my previous post) possibly being something more than friends with Mamoru Kodai. I wonder if his death affected her judgement when the general ordered her to continue the Izumo plan underneath Capt Okita's nose.
Man the wait is going to be so worth it in June when some sites have chapter 5 eps up. If they follow standard procedure for mutinies, it would normally be after a court martial, death for all conspirators or just death for the ring leader. Of course Capt Okita will probably just assign brig time for all of them with Niimi, if she is the ring leader that called for it to happen, getting the longest sentence. We will have to wait and see.