I am not 100% sure to understand all the points of your question, but one thing is for sure, in order to clarify things discussion should be about characters.
An account is intended to be able to allow control of up to 3 characters (number have moved in the past and can totally change again).
To control a character an account must have an available soul and a spark of life.
An account can only allow one active character at a time, if it has more than one character controlled, the others are in OPC mode while the active one is played with.
Exposition is an exception, so far we will only be allowed one character per account during exposition {the one with our surname and titles} (this may or may not change)
So, if we apply all that to the situation you describe :
Comes expo your first character will be your mayor+ one with the surname reserved for your account.
If you want to play a second character during exposition you will need a second bloodline+ account, that character will then have the surname reserved for that second account.
If you want to play a second character after launch (once expo is finished) you can do that either by:
buying an other Elyrian+ account. if that second account has acces to a surname your character will have that surname, if not your character will have the surname of the family it spawn in ( the surname of the NPC you use your soul and spark on).
or by using an other soul and spark from your mayor+ account on an NPC, hence controlling a second character from that account. That character will only be able to get the surname of the family it spawn into ( so if you want both your characters to share the same surname, the second one must spawn into the family of the first one).
What are the main differences between two characters on the same account and two characters from two different accounts?
From the same account:
- both characters can not be active at the same time (the non active one are in OPC mode)
- both characters can access the same account inventory ( objects like pets, mounts and such bought from the website shop and Story Points) {As you can only have one character in expo EP wont be shared between characters because at the end of expo when you can have a second character EP wont exist anymore}
From two different accounts:
- both characters can be active at the same time if you manage to find a legit way to have both accounts running the game at the same time.
- both characters will have distinct not shareable account inventories
Hope that gave some answers.