Are backspace and enter able to be hit easily?
Yes and no.
Yes, compared to their original position on the TM 2030, because with the new position I can press Backspace and Enter without efforts, in particular without moving my wrist.
No, because I can never hit them mistakenly. On the contrary I make many mistakes missing them when I'm in the typing mode. (For example, trying to press Backspace, I hit the , key instead.) For the Backspace key, in addition to using the middle finger I've tried other two promising techniques:
-- index finger: would be excellent if the key was shifted by 0.5u to the left, i.e. the Space bar would be 0.5u shorter. This would require physical modification, though. For the current physical layout, the key is hard for the index finger.
-- thumb: would be very good if I was a right-handed man. But I'm a left-handed man so I never use the right-hand thumb in the typing mode. So my right-hand thumb is not quick enough for my desired speed of pressing the Backspace key. To verify this theory I've also remapped the symmetrical position, i.e. the Shuffle key on the original TM 2030, as Backspace and tried to use it with the left-hand thumb. Works perfectly.
The problem is normal because my thumb/fingers are not trained enough for the new, unusual, position. I've never used a keyboard that places something useful for the typing mode at that position. I believe with a bit more training, the problem vanishes because the position is principally easy.
In the editing mode, I found no problems with Backspace/Enter, as both keys are under control of the index finger.
Generally speaking, no. We aren't familiar with OP's typing technique and physiology though.
Stabilized keys for arrows are a problem IMO, so is the quote placement, and I'm quite wary of destructive functions too close to something non-destructive with the opportunity to make mistakes.
I use standard home positions (if I understand the term 'technique' correctly) in typing as well as editing mode, i.e. Spacebar-JKL; for typing and Ctrl-Left-Down-Right arrow for editing.
I'm a left-handed man, I have an un-trained right-hand thumb and I have rather short and weak fingers compared to many my friends. The right-hand pinky trouble me most as I can never press Backspace with it on a standard keyboard. That's all I can remember about my physiology.
The quote key (new) position is as good as that of the open bracket key and this is the best position I can find for it without re-locating other character keys, which is the last thing I want to do.
The 2x vertical keys for up and down arrow look really weird but that's the only choice to preserve the inverted T shape of the arrow cluster immediately next to the triplet Ctrl, Shift, Alt which is IMO critical for easy re-training from the standard keyboard, not to mention that the vertical arrows are used substantially less frequently than the horizontal arrows, at least in text-oriented applications. Regarding actuation force, I found no noticeable differences to other 1u keys. I can press them easily even by the pinky, not to mention that the TM 2030 is one with stiffest keys I ever use. (70g, if I remember correctly.)
As for the safety of destructive keys (Backspace, Delete and Insert if I understand correctly), I can be certain that at least I never hit them mistakenly in typing mode although like everybody else, I'm completely new to this layout. And in the editing mode I'm yet more certain because this layout is pretty similar to typical laptop/mini keyboards. The Noppoo Choc Mini that you have mentioned previously in this thread, for example.
