Couple things...
First off, Spontaneous Magic can indeed succeed at 10 below the level of the spell like Formulaic Magic, can it not?
And, if so, does rolling in that range gain a Fatigue level (and an extra one if the magus exerts himself?)
Next, although I'm relatively sure I know the answer, can a Creature's powers be countered like a hermetic's if the Form is properly guessed?
For the first part, no. You have to declare the spell's level before attempting to cast it, or find out what the minimum level of the effect your attempting is. So if the spell's level is determined to be 15, your final casting total needs to be 15+ for the spell to work. Anything less either produces nothing or the effect is only in the ball park of what you were attempting and effective in the way you need or want it to be. The reason Formulaic magic has that 10 point buffer is because it's practiced so much both in the lab and in the field, and typically the main reason one fails (or is fatigued) when trying to cast a formulaic spells is because there are poor circumstances effecting the magus, not because they lack the power or skill.
For the second one, yes they can, typically by Fast Casting. Sometimes it's not too hard of a guess, a fire breathing dragon would require Perdo or Rego Ignem usually. The main problem is guessing it, Hermetic Magic has certain tells that all Magi are familiar with, other wizards or creatures with Might it's not so simple. But if it can be figured out you might be able to save yourself.
If the magus exerts himself, he looses a fatugue level, the roll whether successful or unsuccesful has nothing to do with this.
If a magus does not choose to exert himself he does not roll at all.
With a spontaineous spell the magus needs to have a total of at least the spell level (after division) to be successful, within 10 doesn't cut it.
The relevant rules are on the bottom of page 81 and the top of page 82
yes I agree with Qcipher here