Mayuri is a very likeable character
but I was never too affected by her deaths, just because I knew that Okarin would find a way to beat it (maybe I didn't know, as animes can often throw curve balls at you, Steins does it a couple of times, but I just refused to believe that the story would be left in such a tragic state of affairs). I was more interested in Okarin's reaction, especially when he is determined to see himself as a mad, calculating and grandiose scientist.
I was worried about the idea of all those time lines existing and going on without Mayuri, where the gang would have to deal with that absence in uncountable universes, but I think it all ties together and sorts itself out.
On Feyris, that was indeed a very interesting spin which I, for one, didn't expect. The characters have a lot of depth below their surface personalities (which is quite rare in all fiction mediums, not just anime), which made the series really refreshing and has put it up in the ranks of my favourite shows.
Hers and Okarin's relationship was interesting too.
And, please, go right ahead. I stole the idea from all the YouTube comments anyway!
It does make a very good potential alert. Clearly audible and adorable.