Service account help

Hi there you lovely people. I am wondering if you can help me out.
At present I am a PS4 player. I have one Square Enix account and one service account. Both me and my partner have characters on this one service account. This means that only one of us can play at any one time, obviously.

Now we are planning on buying a gaming PC soon and then buying a PC copy of the game. The plan is to add a second service account for the PC onto my existing SE account, and then for my partner to create a new character on there. This would allow us to both play at the same time, albeit on different platforms.

So to break it down: My character will be the PS4 one on service account 1. And my partners will be a new character on the PC using service account 2. My understanding is that this will allow us to both be logged in at once, as we are on different service accounts, despite being logged in to the same Square Enix account. Is this correct?

The other thing that I am confused about is this: Will I also be able to log in and play as my character on the PC? And will she be able to login to her character on the PS4? Because we are planning on rotating who plays on what platform.
My worry is that if I try to login on the PC it won't let me as my character (on service account 1) has a PS4 licence key but not a PC one (as that is on service account 2). And vice versa with my partner. Does this mean that we will have to buy 2 copies of the PC version, so both service accounts have a licence key? As well as another PS4 copy. Because that will get expensive.

Lastly, will service account 2 need it's own sub? I hope you guys can help me because this is all very confusing and I haven't been able to find any concrete answers so far. Thanks in advance.