In the first video I can't really judge how well you're doing (assuming a reasonable and legitimate push from your uke) because I can't see your feet. It appears that you have a slight lean, though. However, all of that being said, words like "relax the chest", "six directions", and so on, don't really tell anyone how or why you can resist a push to the chest.
Suppose that you were on rollerskates... could you resist the push? I don't think so, so (assuming I'm correct), there's a lot more to the story than just "relax the chest", etc. What do you think the rest of the story is?
I think the rest of the story is ... practice.
This vid was done about a year ago and yeah, I leaned. The force coming in was decent but not what I'd consider strong. Hard to tell with video.
In the second video, which is truncated, you appear (to my eyes) to be duplicating what Chris Hein showed, at first. When you turn to the chest push, my feeling is that the push from uke changes, although of course I could be wrong. But the same question arises. How does it work? There is an incoming force of a certain force magnitude and direction and if you don't move then the force magnitude and direction must be met by a counter-acting force and direction, right? If you don't agree, please tell me. If you do agree, then the question becomes one of "how do you generate the counteracting force?".
Again, as I said a few years ago, all you need to do is figure out what it takes to counteract the incoming force and meet it with an equal and opposite force (Newton's third law) in order to put the system into equilibrium. How do you generate these stasis forces? With intent. How do you do it the next step up, in dynamic rather than static situations? With intent and practice and using the hara to manipulate the forces within a trained/conditioned body.
Static forces are easy.
I.e., "aiki" is more complex than simple demonstrations in reasonably static situations.... *and* there are levels of ways to manipulate the dynamic forces.
I generally don't think about counteracting forces. Yes, they have to be there, but I have a hard enough time keeping intent and structure going let alone trying to figure out how physics is working in my body.
That's a headache for sure.
But, I definitely agree on your last paragraph.