That is certainly one interpretation, but it fails to account for the ability to instantaneously alter a person's mind. All other tesseract based technology we've seen has been purely based around energy generation.
The staff being the mind gem on the other hand, accounts for this and everything else just fine.
The original owner (in my assumption) simply knew how to manipulate it's energy in that form. The tesseract is also capable of (and probably not limited to) making energy guns, bombs, force fields, teleporting individuals, and opening portals. It also has the ability to burn through certain metals on contact but not through living tissue so it may have properties that react to the living.
It didn't "make" energy guns, it just powered them. This was explained fairly explicitly in TFA. Just like it powered every other device plugged into it, like Zola's bombs, planes, Selvig's force field and Stark's arc reactor.
The energy is just a by-product though. The cube's function is as the "space gem", it creates wormholes across the universe. This requires energy. LOTS and LOTS of energy. Enough energy to punch a hole in space-time and keep it open. The fact that it gets hot enough to melt steel while at full draw shouldn't be all that surprising.
Maybe he did. Isn't Thanos 2000+ years old? 70 years might be like a few months to him.
A fair point I suppose. Maybe he figured it's nice and safe on a primitive backwater planet.
If that's the case then maybe it was Thor's little adventure that got him moving on it. If Asgard was taking an interest in Earth again then how long before Odin notices the tesseract is no longer hidden and decides to take it back out of Thanos's reach.
I'm still wondering where the Red Skull ended up. Did he pop out where Thanos is? I really don't think he was killed, but transported somewhere. Maybe he was why Thanos knew the tesseract was on Earth.
Could be anywhere really; the universe is a big place. Assuming he's not in the stinger for GotG (which wouldn't surprise me) I'll just assume that Marvel have no immediate plans to bring him back just yet, but made sure the door is left open should the need arise.