I'm not sure what you mean by a 'snap' punch, perhaps this just means a fast, straight strike?
First of all, if somebody becomes aggressive verbally first then you have to think about distance. Be either too close to hit or too far away to hit. Usually people can still strike but trying this it would make it more difficult for them and less likely that you will get really hurt. I understand that a good idea when moving close is to put your hands at your waist, just above their hands so they don't sense you touching them but you can feel and stop their hands coming up.
This is not aikido in the real sense but can be valuable against the 'sucker punch'.
Often, self-defense comes down to basics and I have known people to employ basic entering exercises to real life with great success.
I hope that through practising tanto avoidance I would be okay with a straight attack.
Finally, as always, it is better to have some ability and not need it than wanting to test yourself and being hurt - I personally wouldn't hang around just to see if I could do some aikido.