OK, I just finished reading the Imperial tech list, here's my comments+suggestions.
I agree with almost all of your interpretations, what follows are the exceptions and not the rule...
- We could make Imperial shipyards decrease happiness in the entire system (they were described as a huge drain to the economy), but this has to be tested so we don't paralyze the Empire in the early game.
- Your Version of the Imp. Garrison sounds like something I'd want to build on many worlds, but I wouldn't really need that many spaceports. Why not make Imperial Urban Pac Centers planet-wide only (with an expensive-to-research system-wide Version later in the tech tree), give them a little cargo capacity and call them Imperial Garrisons ? We could build troops and keep them "in" the garrisons.
- Don't worry about building the DSII in six months, that was the Emperor's personal project and he had Vader to hurry it along too. We won't be able to simulate that in SE IV any time soon (Mega-Emergency Build ?). We could still give the Imperials bigger shipyards so they can crank out ISD's, maybe they might have to research Ship Yards and Large Scale Construction to get them.
- I'm designing the Beta-class assault shuttle and the Zero-G Stormtrooper right now, I'll post them when I'm done.
- Please, please give us the Star Wing gunboat and the Missile boat !
- I think the AT-PT was a Republic era weapon. The sequence for Imperial troops could be : Naval Troopers, Stormtroopers, Speederbikes, then (under the Walkers tech tree) AT-PTs, AT-STs, AT-ATs. I'll post my stats for troops and troop weapons & we can go on from there.
- Careful with the mines, they're a major part of standard SE IV game balance. Laser-satellite 'mines' would get lumped together in combat and become ineffective.
- I'd leave out the solar panels, reactors already fill this role. Besides, as you said, TIEs are supposed to be short range craft.
- Imperial boarding parties could be called "Stormtrooper Boarding Squads" and have a little cargo capacity so you could actually carry troop units "in" them (I'll work on the numbers for this & for regular cargo bays, will post them later too).
Hope it helps.