there are too many potential variables, you have to have an interface.
Not true. You cast the spells at the strength you want the crystal to remember them at. Using other buttons to adjust area of effect and what not. When you're happy with it you release the spell button and if you want to add more spell you assign the spells to your mapped buttons and go through them one by one.
The spell in your inventory would be cast at the level that your spell was assigned to during its creation. So a charged fireball would be cast only as a charged fireball. That is you cannot increase your custom spells' power anymore than in its creation. The time to cast it would be the same and would stack with the time to cast other spell effects. There has to be a penalty otherwise its overpowering. So duration of the custom spell depends on how you use it in-game and how much available mana you have.
Within the spell-making circle and in spell-making mode your mapped buttons or keys would take on some different functions. You dont need to run, to strike with a weapon, to pick thing up, to really move your character at all and so on when making a spell in real-time.
Instant cast spells are game-breaking. No one should be able to cast a Master level destruction spell with others spells combined with it instantly at just the cost of mana. The NEW RULE is that spells that are more powerful take more time to cast. This rule is realistic.