Haven't been into any knife fights myself. But I have had 2 friends who only saw techniques from chudan tsuki (typical stab thrust) and were able to apply techs like kotegaeshi and tenkai kotehineri (sankyo) successfully during robbery situations (mind you, these were folks who only watched Aikido and played around with it, not folks who ever stepped on a mat). In both cases they ended up unknowingly with the knife in their hand. Of course in these cases it can also be said that the intent to kill may not have been there with the attackers, which may have aided my friends in not being skewered.
To the points noted above - 1. Knocking the knife out of uke's hand (i.e. kick, punch etc.) may not be a good idea in my view. Have trained with some experienced knife fighters in the FMA and military/CQB world and have found it's a good way to lose a limb, or at least get some very deep cuts on major veins/arteries in the arm or leg if the knife wielder knows how to really use a knife.
2. Taking the knife in the midst of technique may be a bit more applicable depending on the situation, but as Mel said, don't be distracted by getting the knife and ignore the need to disrupt the attacker's balance thoroughly and to keep an eye out for other attacks. Immobilisation and control of the weapon arm is key here I think. We practice this a lot in modified tanto randorigeiko where a uke comes slashing and thrusting at you with full intent.
3. Personally I think this is the best option. It is best to negotiate the removal of the weapon on your terms, i.e. when the attacker is fully under control. In the case of techs where an arm is not free to take the weapon, I say cautiously switch pinning techs to allow a hand to be freed. In techs like kotegaeshi I tend to pin uke on his chest with arm across back and then kneel on it, freeing both my hands to do other things.
Before any technique may be applied though, I think it's important to understand deeply how ma ai, interception/timing and kuzushi work to successfully evade and thwart a knife attack using Aikido.
Luckily, the full force tanto randori we do tends to service this element quite nicely in my book. It's here one gets a feel for how sharp and quick a knife stab just may be, how easily fingers and thumbs may disappear and how hard it is to avoid if the other person fully intends to stab you.
All fun though.
Just my 2 cents.