A 5 foot radius sphere touches 4 total squares, not 9. It originates from a point on intersecting lines, not the center of a square on the grid.
“If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or Attack roll that involves Strength or Dexterity, and you can’t cast Spells.”
Yes, you can cast other spells or attack with melee or ranged weapons while concentrating on another spell as long as the other spell isn’t a concentration spell.
Casting another concentration spell, taking damage (DC10), or being incapacitated or killed stops a spellcaster’s current concentration. Environmental factors can also cause concentration to be outright broken or result in a Constitution saving throw (DC10) to maintain concentration.