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Just found this design and I think it's totally awesome. The new super-small switch PG1316M is used to fit 3 rows into such a constrained space.
Wonder if you could even get away with 4 rows of this, especially to somewhat compensate for the lack of thumb keys? I'd personally be interested to try 4 rows even for a more conventional tabletop keyboard.
EDIT: I have accidentally erased the main URL when I tried to upload a picture. Apologies. (Didn't know Lemmy works this way). Here's the main URL for this thing: https://www.reddit.com/r/ErgoMobileComputers/comments/1npnj85/btyp_a_mini_pg1316m_switch_keyboard_for_use_on_a/
None, just use layers. There are 4 keys with a frequency of 0.15% or lower: https://en.wikipedia.org/wiki/Letter_frequency For a text of 1000 symbols, on average, that would be less than 5 of those characters total.
That being said, the requirements for your muscle memory increases, yes.
For those that don't know what Layers are - please google the concept yourself; if I try to write it spontaneously in my own words, the result will be subpar.