As to the collapsing issue:
AFAICT by looking at your table, IE does not collapse the 'td' wrapper of your child table div wrapper.
I think that it has something to do with the 'visibility' value.
You can apply one of the following CSS styles in order to try and fix your problem:
As per the other issues that you have mentioned, I cannot help at the moment, since I am not familiar with the way in which the nested tables are rendered (they seem to be rendered on expand only, probably not handling the resize/order events).
.x-grid3-row x-grid3-row-collapsed .x-grid3-body-cell
Probably some tweaking is required for the plugin.
I am also a bit concerned about the way the sub-tables are destroyed. It could lead to memory leaks (I am not 100% sure about this, as I haven't dug deep enough).
I might take a closer look at it in the near future.
If I do and in case I get to solve those issues, I will try not to forget to leave an update here.