Citing that law as a reason why the setting that Dioxine created can't work like Dioxine intends it to is weird. It's not like it's a law of nature either (unlike say.. Newton's laws or the laws of thermodynamic). There are plenty of stagnating sci-fi settings and the Piratez setting does seem like a decadent empire. Where XCom is typically optimistic (aliens invade, humans band together, learn all their tech and kick their butts), Piratez is quite a dystopia and nothing progresses much (technology is still generally inferior to alien tech from XCom:EU) besides the gals who are developing stuff very rapidly.
It's important to keep in mind that I am not asserting there is no way the Ethereals could be locked into technological stasis; I am saying that the Ethereals almost certainly would have to commit to a policy of stasis on some level to preclude any kind of significant technological innovation after 600 years given their tech base, and the Empire's evident stability (which presents a conundrum; if you have peace, you have all the resources and opportunity needed for advancement barring specific policy to the contrary; if you have war, you _must_ advance unless your opposition is markedly inferior). It's something to consider when shaping the lore of the universe, and making it consistent/believable.
As for the mechanical side, yes, I think if spitting were to be included, the AI would need a tweak such that the spit is _only_ used in cases where there is no pathfinding. Alternately though I suppose you could engender a fluff reason for impregnation occurring via the spit; i.e. the substance is riddled with eggs, with one ultimately developing when physiological conditions are 'right' and the host body has been adequately subverted with venom. On the flipside if this route is taken I'm concerned about Chryssalids devolving into better Celatids that use their spit in exclusion of their melee.
I agree that changing the Aggression level to 2 for the mindless/aggressive meleers would probably constitute an improvement given the ridiculous behaviour engendered by Aggression 1.
Law of accelerating must hit the ceiling at some point unless a civilization develops and keeps developing intelligences surpassing their own brain power. From what is known, Ethereals did this only once to create brains in jars. Also such a civilization must keep ever-growing records and research facilities.
Also, to innovate upon something, the tech in question has to be understood. If the divide between understood tech and maximum tech is too wide (intermediate techs are lost), innovation is impossible.
It is not clear if Ethereals and their brains understand their own tech. It is fully possible that original creators of their tech are dead or no longer in possession of required knowledge.
If the Ethereal tech is fully understood only by their 'priesthood', and any external knowledge is supressed, it also blocks improvement.
Another of endless possibilities is commitment/understanding of only select areas of high tech (probably mathematics in the case of Ethereals).
The cyberbrain strikes me as the sort that's capable of improving on the tech that exists in 90s if it were willing; though we don't know exactly what it is besides some biomechanical hybrid, if it had say quantum computing capabilities, it is almost certain we would see _some_ manner of material technological improvement, even if it opted not to self-improve, or create intelligences that were superior to its own. Clearly it is not at the horizons/limits of what it is capable of since mere humans were able to reverse engineer and master all of the technology save Elerium. Even if the governor did not possess a complete understanding of the tech at its disposal in the 90s despite being able to fabricate it from scratch, 600 years for an entity like it is more than enough time to capture that understanding and improve upon it.
I get that you're probably going for a WH40k approach; if so, keep in mind that 40k _does_ feature a policy of stasis among the Adeptus Mechanicus in terms of technological advancement at the bleeding edges, albeit one that's inconstantly applied except for certain key concepts (like sentient AIs), with technological and scientific progress ultimately occurring, albeit slowly. That said, likewise mirroring how certain factions and subsections within the Machine Cult advance technology per their independent initiatives, the major powers might feature areas they conduct genuine R&D, with the Church having a keen interest in alien genetics, and perhaps even splicing it with their own in order to become 'transcendent' (on that note, Ethereal/Human hybrids would be hella-sweet).
The bottom line however, is that even if accelerating change proves unsustainable or competing theories prove accurate (and honestly, I very much doubt that they will with the advent of large scale quantum computing), it is almost certain that there had to be some active policy or edict in place to effect the sort of 600 year stasis seen in Piratez, because there isn't even any kind of significant linear advancement, nevermind exponential. Even if you would argue that there's a 'tech embargo' or localized exclusion of advanced Stellar Empire tech with respect to Earth, the same would not be true of the Governor's crib in Cydonia which should be replete in the best it has to offer.