Re: Casting Focus
I mentioned the concern about people feeling like they would be justified in starting with a higher focus because I think you would run into that issue. (I certainly saw your rule, that is why I brought it up.)
Basically, your rule relies on a completely arbitrary decision on 'when' to consider the beginning of an encounter. If a player believes they started gathering focus as soon as the party entered the haunted house and expected trouble, and the GM imagined it was when the Orc opened the side door into the foyer. Obviously... the GM wins, but at the cost of the player's being pushed back from a ease with the game. You system relies completely on that arbitrary point.
I'm sure if presented with that rule... in a published game, my brain would immediately see it as problematic, and internally inconsistent from my perspective, so I would almost immediately begin trying to find an adjustment that would make more sense to me. Something that would better explain the thresholds and their implementation.
Would the following potentially be workable?
Casting a spell requires bringing a spell to mind, as well as having gathered magical energies and being prepared to focus into that memorized, and brought to mind pattern.
In order to bring most spells (ones with at least 1 standard action as a casting time) normally take a basic of one round, per level of the spell before it is 'ready' to be cast. Spending a swift, move, or standard action will reduce this time by one round each action taken.
Your casting attribute, among several thing controls how good you are at being able to understand and manipulate things. This means that if you change the spell you are focused on, it determines how many rounds of preparation originally used towards your old spell may be re-directed toward your new spell focus.
There are some spells which you can cast without completely losing focus on your currently brought to mind spell. Cantrips are one example.
Spells which are not the highest level spell you can cast for the class and are ones which your casting attribute gives you a bonus spell for, are easy enough for you to bring to mind, and cast, without completely losing focus of your original spell. Casting such a spell only sets you back the number of rounds equal to the level of the spell cast.
Spells which the class has the ability to spontaneously cast out of another slot. (not all spontaneous caster) have the ability to cast a spell allowed by that class ability, and that ability to spontaneously convert can be used again like a second 'focus' slot after the number rounds equal to the cast spell. (so a cleric could choose to cast a Cure Light Wounds, without affecting their focus of another higher level spell, and could cast another spontaneously converted Cure spell next round if they pleased. Druids could do the same with their summon spells.)
Swift spells don't have to be brought to mind and focused consciously (some of the extra magic required to cast them, making them higher levels, is actually used to bind them to your mind in a way that they are quickly accessible)
Casting a spell clears the focus from your mind, so even if you had two fireballs memorized, after casting one, you would need to refocus to bring the spell back to mind.
Net result, a wizard might be able to be prepared to cast one of his higher level spells at the start of an encounter, but would need time to refocus if he decides he needs to change which spell first.
The spending moment to reduce the time until achieving necessary focus for a spell is applied to a particular focus. If a spellcaster casts a 3rd level healing spell spontaneously, and they decide to use two actions to regain focus, the following round they could cast another spontaneous cure spell, but those actions don't add to the other spell defined by their normal focus (although they could have applied those actions to that focus instead of the spontaneous focus).
Anyway, just a thought for you to mull over. It would make your next spell choices much more strategic, and additional requirements of time between spells to make protecting the mage more important.
Otherwise, as I mentioned, I think that you would need to define some 'limit' of how long someone could maintain a spell focus over their highest casting attribute. Past say one minute, they would have to make some sort of fortitude roll or become fatigued. Then you would have a mechanic, where is someone wants to 'as a player' mark a particular time, they can, and work to get their focus up to a higher than usual level, but if they do it too early, they risk becoming fatigued. This would make your 'normal' static situation of starting with that value of focus the 'standard'.