One of those areas where the rules had been 'fudged' somewhat, dare I say, like wards (ARRRGHH) to try and maintain older edition spells in a comparable level effectiveness whilst continuing to use the same 'spell building' process as other spells in 5th.
Oh and I freely admit it took me a while to work out the maths in my head, what you meant about a dodgy lvl 3 version 
Nope. You need to cast a rego or perdo vim spell to finish an other spell prematurely, or have a special virtue (like those found in the Mercere chapter of HOH:TL) to do so. Merinita dudes, with "until XXX" duirations can also work here, but there is no way to stop a spell before that without those special abilities. You can dispel it. That is the most similar thing you can do here.
I think you got it right, but fast cast defence is not something we use most of the time, since the fatigue you suffer is prohibitive to magi most of the time.
wait for more knowledgeable people to ansswer you here 
I would also allow Creo Aquam to counter the enemy spell if it is fire, or Creo Ignem if it was causing darkness or cold.
Compare it with Mu(Re)Te10 Crystal Dart. +10 damage. No aiming
Base 3, +2 voice, +1 rego
Pilum of Fire (CrIg20) is not aimed either.
Your spell would be fine as a CrAq better than an ignem spell IMO. The range needs to be superior to personal, or you can only dfamage yourself with it!! 
DFor what you want to do, range voice would be more appropiate. The range is how far away the spell "flies", not where it has originated
For a plain +2 damage, a base of 1 or 2 would be enough IMO. I do not have my books to check, though.
Cheers,
Xavi
The shapeshifting ( called transformations, since with different requisites you can become other than biological 'stuff' ) guidelines under MuCo -do- give you an exception to that general rule of not being able to cancel your own spells without some kind of special intervention. ( Which is accurate. ) ArM5 pg 132 has the guideline. It is in the second to last paragraph on the right that it describes the need for foci and how they are used to accomplish this early ending of the spell.
So while you 'can' end a MuCo shapeshifting early, like the heartbeast, you require a foci to do it. Which makes it not quite as good as a heartbeast still. Not to mention the heartbeast doesn't generate warping for prolonged exposure to the 'effect'.
Bigger is not always better, esp with Durations. And once you set a magical force in motion, it continues until another (magical) force acts upon it, or that Duration ends.
"Dur: Concentration" is the only way to have such control, aside from obscure Virtues & etc.
A close second is the series of PeVi spells "Unravelling the Form of (X Form)" (p 161). (If Penetration is required, that's where your suggestion of "cancelling one's own spell" might come in.)
Heartbeast is not a spell, and is superior in many ways. If an Intellego spell is cast on your bird, the spell detects a human in magical bird form. If on a heartbeast bird, it detects a natural bird.
The character has to get creative, and achieves the effect they think is best suited to the situation and their Arts.
Let's say a big Pilum of Fire is coming at your mage - big hairy column of Ignem unpleasantness.
PeIg - stop it in its tracks, or reduce the heat/intensity.
ReIg - deflect it (but it still goes "somewhere"!)
CrAq - create a water blast to meet it halfway - and results in steam
CrTe - create a stone wall - cool!
ReCo - teleport your ass outa there!
...and so on.
But yeah, you have it about right.
(Don't think "Vim" would work well, except at prohibitively high levels.)
Ice is traditionally covered by Aquam, not Ignem.
In 5th ed, a spell always shows up where you put it - but that may not be where you "want" it. With Aimed Spells, you are trying to put the spell near something/someone else that is your final goal - creating a hole in the ground under their feet, for instance, or creating a stone to fall on their head, or a thorny hedge around them. The hole/stone/hedge appear exactly where you cast it - whether that other person/thing is where you thought they'd be is the question.
If you are Rego'ing something at someone instantaneously, it hits - the magic and timing combine to guarantee it. If you are Creo'ing/Perdo'ing something that is near something else, or Rego'ing something like a sword over a period of time, then it's aimed. (Many "aiming" rolls are dirt easy, others are quite a challenge.)
See spells like Crystal Dart vs Teeth of Earth Mother (both p 154).
The others have commented on the spell relatively well, so I in my great wisdom don't have to. One thing though. The above hints that you might not understand one of the essentials of Ars Magica magic system yet:
Creo Ignem spells create either heat or light. They don't create cold and thus they cannot create ice. If Ignem was used to "create ice", it would be more like "destroying heat" and thus belong under Perdo Ignem arts, but like Sodalis Cuchulainshound said ice belongs under art of Aquam. I hope this is of some assistance Sodalis Mintroll.
Ice is aquam mostly - but you can also create it with Perdo Ignem as long as you have something, water, to freeze into ice, just as it can freeze anything else solid.