Hey Jean...sure make me think on this fine Monday morning <grin>
WRT the toes thing.
I think that "gripping the mat" is more of an image thing than a reality thing. I understand it to be a way of bringing your toes in close together so that they can move together and not have one or the other get caught on something. If you do actually grab the mat with your prehensile toes, you'll just have to let go when you go to move. So...it should be easier on a hard surface because you won't have anything to get caught on as you bring your toes together. It will also help focus your weight on the ball of your foot and not your heel - or else we might end up turning on our heels and that would be bad.
WRT multiple uke's and forward motion.
It might cause you to have a deeper stance. I find that it makes me use my forward knee more so that I can get to the balance point where I can push forward and under/around both uke's power points. It is probably a little lower than with a single uke.
I find that this balance point is the same point where I can start pushing with my front foot and allow my back foot to cross-step. We do some suriashi practice where we have to cross-step with our feet sliding along the mats and for resistance have someone holding onto your belt from behind and pushing your shoulders from the front (FYI...Robert Sensei calls this Japanese Ice Skating). The importance of a strong stance and the use of the front knee really shows itself here.
WRT the lines between uke and how to follow them.
I think you're right in that if you try and get around one uke you will probably end up in the center of the other uke. For me, I try and see the line between the two uke's and follow that. There has to be a point between the two uke's where there combined force is weakest, or maybe most balanced between each other and that is the line you have to follow. Its kind of like a vector equation from high school physics <grin>.
Anyway...I would say don't think about just one uke in this, but think about where you feel your own balance and power and finding the place where its easiest to move. "Easiest" in this context might be relative, though
For me, it really helps to imagine and visualize a line in between whatever forces are being applied that I have to follow.
I really hope this makes sense. I am basically regurgitating things that my instructors have told me, at least that part of it that I think I have some understanding right now. If you ask me the same question next week or next month or in 2012 the answer might be different. <sigh>