Meta E4 Full Loop Interview Experience! (Awaiting results but not positive)

Hi all! Just following up on my full loop experience with Meta and giving back to the community as much as I can BASED ON MY EXPERIENCE. I will skip the behavioral round details as it's fairly standard! Now to the meat of it..

C1Q1: 346. Moving average from data stream Instead of implementing the "next()" function, I just add to an array of the sums by the time I get to the last element of the array, and return the resulting array. Ex: Given a list [1,2,3,4,5] and a window size = 3, my resulting output should be [2.0, 3.0, 4.0]

C1Q2: 215. Kth largest element in an array (no variant) I was under the impression the time complexity for a MaxHeap here would be O(nlogk) however the interviewer was convinced it was O(nlogn). Not sure who is right because when I use leetcode's analysis tool it tells me O(nlogk).

C2Q1: 921. Minimum add to make valid paranthesis (no variant, nothing more than paranthesis on the input string)

1. Random pick with weight (variant. At least, I assume this is a variant because it seemed very similar to the original question) Given a list of cities and their respective populations in a map (ex: {NY: 7M, LA: 5M, BJ: 10M}) create a function that randomly picks a city based on the probability that a random person would be from that city (so for NY it would be 5/22, because it has 5M out of the total 22M population in the map).

System Design: Design a scoreboard for some online (or mobile) game that shows the Top 10 rankings of my friends, and globally. It should also show the top rankings of all my friends and all globally (not just top 10, I should ask to better define how many I am willing to show, how to show it, etc.)

I believe I did well on everything, except for the coding interviews. For all of the questions, I talked through the sub-optimal or "naive" solution, talked through and agreed upon the optimal solution with my interviewers, but ultimately could not code a "bugless" final solution, even though I've practiced these questions ad infinitum. Nerves just got the better of me!