Hi,
For all three of your questions, there is one answer:
With HikaShop 4.2.3, we've added a new feature to the characteristic selection so that the dropdowns of the characteristics are dynamically modified to display only valid choices so that choices without stock will be removed automatically. To do that, it requires to hide all but the first characteristic selector so that when you choose something in it, we can calculate what are the variants with that value that have stock so that we know the values we need to display in the second dropdown etc.
And only after you select all the characteristics can we refresh the image and other information of the product and display the add to cart button.
But we've added an option called "Characteristics dynamic display" in the HikaShop configuration so that you can just turn it off and it will revert to the previous characteristics selection mechanism where a value is automatically selected in the dropdowns and thus the information can be directly updated and the add to cart button can be directly displayed.