Question 1: Yes

Question 2:You will then be able to unlock a quest as your LVL 30 THM to embark on becoming a BLM; the quest is located in the THM guild after getting your archer to lvl 15

Question 3:No Your BLM will start at whatever level you left off on as your THM as long as its over 30 obviously

Question 4:Yes/No as certain classes require two main stats i.e as a WHM you wouldn't throw all of your points in MND as PIE plays a part in your role too , as well as dps classes placing a little in VIT just for survivability purposes. End game when you're fully geared allocating all 30 in your main att. would pose no problem at all but leveling it requires spreading around minimal yes, but it still requires a bit.

Question 5:It will still use the ATT from the BLM: A Black Mage is a THM not an Archer so it will follow your base class line