Heimdall
The borg.
The borg - sentient or not?
Once again - during star trek, the borg were described multiple times as being a giant consciousness. Consciousness does not mean "an entity with perceptions", it means sentience, awareness.
For example (and there are others) - In VOY:"The Gift", Janeway describes the borg as being a giant consciousness - sentient in the fullest sense of the word.
This means, of course, that the borg are evil - by any definition of morality: deontological or utilitarian.
Defending against the borg - self-defense or not?
If you know beyond a reasonable doubt that your future aggressor will try to kill you, you have the right to defend yourself.
For example, if your agressor is a professional assasin and you know he was paid to kill you in an hour, you are in self-defense when, lacking other options, you are attacking him, trying to stop him from killing you.
The virus from "Q Who" - will it work?
The scenarists' intent was clear - if released, the virus will work. In the episode, there was never a doubt about that. In future episodes, the scenarists reinterpreted their ideas - somewhat.
In VOY:"Endgame", though, that neurolitic agent Janeway used to infect the borg worked like a charm.
Picard&co had only a slightly better understanding of borg tech than we do?
No. Absolutely not.
We have no understanding of Federation technology whatsoever. Or, perhaps, you can tell me how to generate efficiently strong gravitational and antigravitational fields? And be as detailed as possible!

The Federates are at a level at which they can understand borg tech without 10 years worth of explanations - unlike us.
Utilitarianism
"90% chance? Ah, based upon what?"
Based upon the scenarists' intent.
The percentage could be a lot lower and still the option to use the virus is the best option.
You don't seem to understand the gravity of the situation. Everybody in the Milky Way Galaxy is living on borrowed time. In a few millenia, at the most, the entire galaxy will be assimilated - unless a mindboggingly improbable event happened.
Deontology
Picard choose not to use the drone because he wanted to make the perfect moral choice, to look himself in the mirror with pride - and he sacrificed BILLIONS (that billions will die is a certainty) for that. No matter how you look at it, that's egoist. It may not be deontological, but it's egoist.
PS - Have to go for today. Thanks for the cool discussion.