Re: Suwari waza
Although my answer, or rather the instance I gave was more kokyu dosa, the suwari waza are an extension from there.
Key differences between that and tachi waza? Well my first thought is none, ideally.However, from the view of learning then there are many things to learn from each.
Suwari waza at first are much harder from one perspective yet surprisingly easier from another. On the one hand they are uncomfortable for those not used to doing anything from the knees. My own view on this is to learn how to move from center, practicing tai sabakis for example until you viryual float around without hardly any pressure on the knees.
One thing it teaches you is that a giant is easy to handle. If you are doing from your knees whilst the attacker is standing attacking then it represents handling a very large person. Weight underside is also very useful here. Short people get to learn how easy it is to bring the other down to their own size rather than be overwhelmed by size.
Anyway, as all has to be done from center and the hips then the reality of such increases, the awareness of alignment increases, the awareness of the basics increases. It's amazing to discover how the 'legs' get in the way of correct movement.
From tachi waza as I said the movement ideally should be an extension of suwari waza. So it's usually what you need to stop doing from tachi waza rather than looking for key differences in my opinion.
From the knees for example you can't do 'too many steps' for if you did you would topple over. You learn to glide more than up down steps.
In fact when I see up down motion through 'stepping or walking' I say that is a mind going up and down, not stable. Same goes for someone bouncing around in front of you 'sparring'.
Thus from standing minimum steps should be used, just like the motion in suwari waza. For example I would discipline someone to do a complete 180 degree tai sabaki which takes you around and behind the opponent to two steps only. Secondly to learn how to glide rather than step thus moving from center rather than head.
So really the only difference I would say is amount of space you can cover from standing in a shorter period of time and that's about all really. Suwari waza and indeed kokyu dosa give you eventually that stability and oneness feeling you should carry through to tachi waza.
Well they are my initial thoughts and responses to you question. Hope it helps.