Hey there Eentje!
Hopefully I can shed some light on the question you had.
You are correct in that if you use the Character Name Change service your name will be unavailable for any other player on the same realm for 90 days. You can still create characters on the same account to use that name during that time.
When you delete a character that name will be reserved for 30 days, but that only happens if the character was level 30 or above when deleted. Same rule applies as for Name Changes, the name will remain available to be used on your account during this period.
I hope this helps!