For play, when need basically two things:

• a simple play that start at current frame to the end (and loops),
• a "quick play" that plays only a small defined amount of frames before/after current position, this could be 1s around current position by default (so 0.5s before and after), with a setting somewhere to change it for those who want more or less..

(-> play selection is not so useful)

About 3-C, maybe switch time to first frame of selection would be better than latest.

About 4-E: could it be shift instead of alt ? also it should be more precisely "shift the frame and all its right-hand-side frames to the right or to the left until the previous keyframe."

Another thing I'm thinking about, with this proposal, it's only possible to select multiple frames of one layer if there are empty frames to start the selection
That's a problem

maybe it can go like this:
-click/drag from a not-already-selected existent frame select it/multiple frames
-click/drag from an already selected existent frame move it/selected frames

New proposal

1. Horizontal header
   1. click: switch time + select frame of the current layer
   2. rclick: menu (insert/remove columns)
   3. click+drag: switch time + select frame of the current layer (scribble)
   4. shift+click: select column
   5. shift+click+drag: selection multiple columns
2. Vertical header
   1. click: select layer
   2. rclick: menu (add/remove layer, show/hide layer)
3. Frames Table: empty area.
   1. click: select single frame + switch time + *don't* switch layer
   2. rclick: menu (insert/copy frame)
   3. click+drag: scrubbing + switch time + *don't* switch layer
   4. shift + click + drag: select multiple frames + switch time to the latest + *don't* switch layer
4. Frames Table: area on existent frame.
   1. click: select single frame + switch time + *don't* switch layer
   2. rclick: menu (remove frame)
   3. click+drag: *move* a frame or multiple frames
   4. ctrl + click + drag: *copy* a frame or multiple frames
   5. shift + click + drag: select multiple frames + switch time to the latest + *don't* switch layer
   6. alt + click+drag: shift the frame and all its right-hand-side frames to the right or to the left until the previous keyframe (stretch the previous frame)
5. Frames Table: all
   1. space+click: pan
   2. space+rclick: zoom