Swedish grammar is reasonably regular; for böjning of nouns you can find information online, for example at https://www.worddive.com/grammar/sv/svensk-grammatik/2-bojning-av-substantiv/
The key principle is that you add "-et" to the end of the word; if it already has ä vowel at the end, then ä transforms into e. Vowels are not doubled. The profession-type words that end in -er or -re work differently, as you can see from the link above.
Another way of thinking about it is that you try to "t" to the end of the word, and add n "e" before that if necessary to make the pronunciation easy (to a Swede).
This and a bit of practice should give you a good handle of the böjning. To verify how a particular word works you can use the two first dictionaries at https://svenska.se/tre/?sok=kilo; by clicking around you can see the böjning of the words it finds.
"Kilo" is indeed a loan word and the correct form is "kilot". To check if this is systematic or not, you can try to find other neuter nouns that end in "o" and see if you can find any and how they work. My Swedish is too weak and rusty to help here, unfortunately.
For general information about diftongs, you should ask another, more specific, question. As a generic answer, start with Wikipedia https://sv.wikipedia.org/wiki/Diftong and grammar books and go from there.