[@That1Guy]

[list]
[*] First and foremost, is there some kind of Magicka we need to keep track of, or do we just base it on how many spells we think they can cast in quick succession without being tired? 
[b]Yes. Player discretion, please. It'd be like if your character tried to sprint a marathon. Imagine the intense amount of concentration one would have to have to cast flamecloak, concentrate on keeping it up, then maintain that concentration to start flinging fireballs, then try to concentrate on pulling the very being of an atronach from its plane of Oblivion and also concentrate on holding the psychic link with that atronach so it doesn't start slaughtering everyone wholesale because 'holy shit where am i this is not home who are these people oh fuck they're all fighting'[/b]

[*] I've picked from spells that come from the games, but how do those convert themselves into this RP? Do the durations stay the same, or are we allowed to fiddle with how long spells can last? 
[b]Again, player discretion, refer to my answers before and after this one as well.[/b]

[*] Can we have a Concentration spell (like in D&D if you know it), which means it is up as long as you can maintain it (usually has a longer duration), but can only have one Concentration spell running at any one time, so should you wish to end the effect or your concentration gets interrupted, you lose hold of the spell. 
[b]I've heard it said that mages commonly cast from the hands because it is the easiest point to focus your magicka. It is fair to say that any spell cast needs a fair bit of concentration to maintain, but this is interpreted in two different ways:

With spells like flame, frost, healing, etc. (spells that one would need to hold the trigger down to cast in-vidya-gaem) it is safe to interpret that game mechanic as the spell lasting for as long as you can concentrate on keeping a steady, even flow of magicka pouring out through the mental focal points that are your palms.

With spells like fireball, fast healing, ice spike, etc. (spells that require one press of the trigger) it could be interpreted as the character focusing on building up magicka at the focal point of their palms and releasing it.

Spells like stoneflesh and other 'mage armor' spells could therefore be interpreted as the mage in question visualizing a 'second-skin' of magicka coating themselves.

So, yes, each spell is a sort of concentration spell in its own right. I hope this helped explain how in-character casting would work.[/b]

[*] Are we able to cast with both hands to make more powerful versions of the spell? If that is need to be learned, can we learn it? 
[b]Yep, the hands are the easiest focal points for a mage to pool their magicka and release it, it's fair game to say that the act of using both hands could mentally lead to better concentration, ergo, a stronger spell effect.[/b]

[*] How many spells can we choose to know if we're Highly Proficient in a school? On Brynja's CS I see seven. How many if we're Somewhat Proficient? [b]Lemme get back to ya on this[/b]

[*] Can we cast "Self" targeted spells on others? 
[b]Things like mage armour require the intense mental fortitude that is trained up by mages in order to visualize that second-skin of magicka coating their bodies. It would take the target being able to employ the same techniques the mage is using, but at that point, they'd be better off casting it themselves. The different levels of mage armour(oakflesh, stoneflesh, ironflesh) could be interpreted as the amount of magicka the user is able to pool up in a 'layer' throughout their entire body.

This same logic could be used for other spells of the same nature, cloak, waterwalking, waterbreathing, etc.

In the case of healing spells, theoretically, yes.[/b]

[*] Can I get a cookie? 
[b]Yea, of course. Funny, weird thing though, I keep them in my basement sooo. Yea, you first, I'll be down in a sec.[/b]
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[@That1Guy]
The best character I've seen played with an Atronach birthsign was another guy from another TES RP. He took the birthsign and really ran with it, having the character's pool of magicka effect not only their casting strength but their mood and energy level. A human battery, if you will. You can refer to my answers to the questions above for how in-character casting is interpreted. Feel free to let me know if you need me to flesh out any of my answers for you.