I'm not at home with the game right now, so I'll touch back on this later.
Not alerting:
I have alternating autosaves every fifteen minutes. I'm not sure if they went on alert while I was currently shooting, but certainly went back to not alert when I was hanging around. A neinzul cruiser can be a cross between a stealth battleship and a immortal botnet golem (extremely fun revenge for all those stealth master AIs), so I'm thinking even if it isn't attack, it should irritate the system it is currently in. I'll load up one of my multiplayer autosaves in singleplayer, so I can pause and review the galaxy map while attacking, rather than when I'm fleeing to the edge of the system.
Extra Difficulty option:
Exactly, the AI gets the EXO, but you get nothing in return... except your champions (I'm imagining battleship if not dreadnaught champions here).
Missiles:
Well, Missiles on Frigates are extremely lackluster, since you're giving up the shields you need for scenarios. On the other hand, slap them on a Neinzul destroyer or team them up with a shielded champion and you're looking quite good.
Leveling:
Speaking of leveling balance... one problem is that a lot of the modules (especially for neinzul) are entirely useless in a nebula, and in core systems. This makes spending unlocks on them a bit of a risk. You have to make a conscious choice between excelling in and out of a nebula.
Leveling slows to a crawl around 20-24, which is fine. The problem is, to advance further and correct mistakes or experiment involves grinding.
An example of the type of change I'd like to see is: Bonus Levels on top of and separate from your existing experience level. Completing certain tasks would give you bonus levels, like 14+5. The key difference is that these bonus levels do not increase the cost of gaining the next natural level, so completing one of these tasks an hour into the game is no less efficient than completing it five hours into the game.
What would generate a bonus level? The best answer would be new missions, of course. Ones that take place in the main galaxy, even. (have to kill a spire city hub perhaps with attendant frigates and destroyers?). A more realistic answer would be: Advanced Research Stations (+1), and Archives (+3/+4). You could replace level 20 farming with these objectives when you need *just one more point* and it would be more interesting than parking/FRDing a cruiser. Of course, I have no objections to high level challenges strictly for the bigger ship classes (since there were hints about varying strengths of nebula jumps... mk IV nebula, anyone?).
The problem is, once the nebula challenges slow down or are no longer available, there is nothing in the universe that allows a decent level of experience to come in. We are talking level 20-24 here, where unlocking a module up to mark iv is very expensive. It is okay to let them slowly level up while doing things around the galaxy, but it is extremely slow to get any rewards at a certain point. And when you only have a champion, you need the feeling of reward more often, rather than turning into a space janitor. Not unlimited rewards, for certain, but ones that reward working towards the normal players end goal of taking out the AI.