Mibox specs are average but storage space are abit low, which requires constant cache/data clearing or Kodi might crash.
The up side is their UI is simple and nice looking, plus the remote is sturdy and works well with Kodi interface.

Pls be informed some other box remote "return/back" button actually return u back to Kodi main screen instead of "1 page back" within the video addon.

1 - 4.... Most reasonably priced boxes handle this well, besides can always install & customize a different launcher. Ease of use? Get a good airmouse or touchpad + keyboard Combo remote..
5... Good luck.. 
6... Same as 5.. I believe the Mibox has better support than HiMedia.. Minix rules.
7... These are boxes running heavily tweaked softwares that were only meant to run on mobile phones and tablets.. Bugs & tweaks are expected. So much here depends on one's willingness to dabble.. The more questionable apps or addons installed, the higher the chances of something going wrong. Especially on a box with lower specs or buggy firmware.

a regular wireless mouse and keyboard mouse will do

u can also consider getting just a simple wireless mouse.

a simple mouse combined with the device remote control will be able to everything and no chance of incompatibility.

I prefer the A2 as it feels more like a remote than a keyboard + touchpad combo.. The latter feels too awkward for me during daily usage. So yeah, while most regular bluetooth or usb dongle wireless device keyboards and mouse will work, for a media center, like me one that looks and functions as a remote while still sporting a full keyboard. So weigh both options & go with which you reckon would be a more comfortable fit in your hands & for daily operations. Airmouse (waving) vs TouchPad (sliding).