1) Yep, you can turn off all double click actions in the settings. I personally use it a lot, for deleting keys and in the viewport to deselect. Some people don't like it though, especially when using a pen, so there's a setting: double click shortcuts under General.
2) Key offset was changed because it was tedious to have to select all the keys each time you use offset. I know the change is a bit jarring for those who are used to leaving offset on, as you can no longer move individual keys with it on. It may be more comfortable to not turn on the Offset button, instead hold ctrl+alt and drag a key when you want to offset. There is also alt for temporary key shift and ctrl+shift for duplicating a key (I use that a whole lot).
Note that deselect (spacebar or escape) will deselect the dopesheet keys, in case it is annoying that offset changes the selection. FWIW, in v4 key offset doesn't change your key selection, though it still offsets instead of allowing individual keys to be moved.
Hopefully the other many improvements we've made make up for these two! 🙂
