I'm not familiar with the 'xkb in the software end', but what you tell above souds good. Especially if you need to write with a character set of a 2nd or 3rd language, or use a lot of special symbols, it is nice to have a special new layer of your "own" characters. Like you say, memorizing them is a challenge. That's why layout designers and localizers want to stick to some standard conventions, and leave the personal customizing to be done by the user's own wish and responsibility.

Yes, there are many factors which affect the writing ergonomics. And they really need to be tested thoroughly. A "Sliding Qwerty Keyboard Half" will certainly be even more challenging than the existing sliding Qwerty phones, because the "display part" (= the phone) will be a lot heavier than the display part of N900, for example. Text entry is affected a lot by the balance (where is the weight center of the phone + OH combination). which depends on whether there is a battery in the Qwerty, and where exactly. The optimal place for the battery could be below the keymat, but such a place can make the Qwerty OH quite thick. In any case, many tests need to be done. Perhaps also the place of the keymat need to be tested; what is the optimal distance between the leftmost and rightmost keys from the left and right ends of the keyboard.

Below you can see the Qwerty of N950. Like on most HW Qwerties of Nokia it uses three modifier keys which affect characters: Shift, Sym and Fn (on this keyboard using the "north-east arrow" label).


With the Sym key can be entered a wide variety of characters that are used by most European languages. Its operation is different from the corresponding "press and hold a key" function of virtual touch-screen keyboards. But their purpose and character sets are quite similar. On HW Qwerties "multitapping with a key while holding the Sym key despressed" is more complex to use than the corresponding "tap and hold a key and slide onto the wanted variant letter" function of virtual keyboards. But in the case of most European languages, the most frequently used variant letter is there as the default letter on virtual keyboards, and as the "1st letter variant" on HW keyboard, which you get by pressing the Sym and letter key at the same time (or the Sym key a bit earlier). These operations are complicated to explain in writing, but once you've learned to use them, you will not long for any "special character tables", which are still found on some phones. The Fn key is needed for punctuation marks, paretheses, etc symbols which are printed on the keys. For these reasons I recommend keeping both the Fn and Sym keys on the keyboard. The Sym key is essential also for keyboard shortcuts: on smartphones the Sym key could be used like the Alt key of full-size Qwerty keyboards in some keyboard shortcuts, such as Sym+Enter (= Alt+Enter). For the planning of keyboard shortcuts, the standard full-size keyboard is the best reference. Because the simultaneous pressing on three or more keys is too difficult, keyboard shortcuts should be used moderately, or they should be made with "sticky" modifier keys. so that you can make the keyboard shortcut "in series".

The Ctrl key is a must, which often saves us in some difficult situations. For instance, selected text can be cut/copied/pasted with Ctrl+C, Ctrl+X, Ctrl+V, respectively - on web pages there often is no other way to make those operations. On touchscreen you may see corresponding function buttons, but when writing text on a HW keyboard it is usually a lot more practical to use the Ctrl-shortcuts than the Cut / Copy / Paste labels on the touchscreen. Keyboard shortcuts made with arrow keys are very powerful. Because there are no keys like Home, End, PgUp, PgDown, some 3-key shortcuts may be needed.