"ī" is a long i while "i" is a short i, so it makes a difference which one you choose. In this particular case, the lengthening of the last vowel in "muñi" to "muñī" indicates that there's an implicit "and" between "kepi" and "muñi".
In other words, if you wrote "muñi" here instead of "muñī", in effect you answered "the fathers the mothers are good" which is not a correct sentence.
The y in High Valyrian is pronounced like the German ü, which the best explanation I heard as to how to pronounce it is to make an "i" noise, with your lips rounded as if making a "u" noise. ȳ is the same, just longer. As to how to write it, in Duolingo you can type "yy" and it will parse it as "ȳ". Same for the other long vowels.
In Ubuntu, if you turn on "Compose Key" using Ubuntu Tweaks package, you can do a typical key combo of (RA=Right-Alt key) RA,Dash,i and get a ī, RA-shift-tilde-n to get the ñ. Other than installing the tweaks package (which extends standard default settings changes), the process is pretty intuitive an simple.
In windows it's a bit different, and in my experience more onerous. https://www.johndcook.com/blog/2008/08/17/three-ways-to-enter-unicode-characters-in-windows/
Mac, I am not so sure, but I would wager it's simpler than Windows.
There is also this site too.. more steps with copy paste included however works with all platforms (Windows, Linux or Mac) and in any site that allows unicode in postings (such as FB and Twitter). https://yaytext.com/tiny-text/