To decide which keys I'll add to my layout, I was wondering what is possible and what can never be possible.

I'm a software engineer, so I can probably add some things myself, but I'm not familiar yet with writing phone apps (I got as far as creating a hello world Andoid app) and I'm not familiar with sailfish at all.

Because I work a lot with TV's, I think I'll be adding the common buttons of TV remotes to my layout. Just in case I ever write a remote control app. Many of these keys (red/green/yellow/blue/ch+/ch-, etc) make no sense on a phone, but is it possible to write an app which reads these keys, and send them to the TV? (don't worry about the sending to the TV part)

Other keys found on TV remotes and many PC keyboard are the media control keys play/stop/ffwd/next, etc. These could make sense on a phone as well, to control media players.
Could this work?

How about volume up/down keys? The phone's own keys can be obstructed by the keyboard, depending on the orientation in which you attach it. Could I add these two keys to my layout and have them work identical to the phones own keys?

Can I add a (working) mute button? Maybe even different buttons for different profiles/situations? One for complete silence, one for vibrate, one for civilized noise levels and one for absolute mayhem.
Or one button which pops up a dialog with the different profiles?

And could I write my own app which captures the vol+/-/mute keys first and blocks their normal behavior? (So when in my remote control app, use these keys to control the volume of the TV set, not the Jolla's volume)

Can I add keys that do something on the phone no matter in which state/app it is? Like:
- Can I add an voicemail / 'ICE' button which directly calls my voicemail / ICE-contact?
(common on many cell/wireless phones where you can hold the '1' to directly call voicemail.)
- Can I add a key to open certain apps (email client, web browser)?
- Can I add a lock button, which locks the phone? Like the button already on the phone. Although the existing button might be very special button, just like the power button on my PC is not a regular button.
- Can I add a power button to power down the phone? To power up the phone?

Can I define an action to execute when holding a button for a longer time (like calling voicemail by holding the '1'), or does the keyboard only register keypresses, no matter how long they are pressed?

Can I create a button for force a certain rotation of the display? I know you can temporarily prevent an orientation change by touching the screen while rotating the phone, but I have many situations where I'm not holding the screen but something triggers an orientation change (like making a sharp turn in my car, which causes my navigation app to rotate). So a key to set the orientation to landscape/potrait/auto would be very nice.
could pressing that key provide some visual feedback (like the android toast message)?

Can I create macro record/play buttons? I have one unused key in my layout, whick I would like as <macro> key: <shift>-<macro> to start recording keypresses a until I press <shift>-<macro> again. Pressing <macro> would replay those keypresses. (for example fot those annoying email-password dialogs that I have to fill in 10 times a day)

I expect this to require some programming from my side, but I would like to know if it can be done, or will be impossible. (any indication on whether this will be easy or very hard is appreciated)