question #1
in japanese the n is phonetic, so the jin sounds like jing because of the ji.
like in san, when you say san, the n is pronounced as "en". however in english this is written as "n" because there are different "n" sounds.
for example: ke is written as "ke" because that's what け sounds like. and ん is written in english as "n" because it sounds different depending on the use (san, jin)
the single letter "n" cannot be pronounced without putting it next to a vowel (an, en, on, in,un)
question #2
from what I know,
things like san, kun, chan and such are written in Hiragana. I think this is because there are no chinese characters for these phrases
the same goes for wo, wa, desu, e.t.c.
I hope my rambling was useful