nice catch Spitfire.
after looking up the information, i concur with your analysis. job's a good 'um
as previous comments has answered the question about the weapons effectiveness and why is human-zentradi "modern" technology is so ineffective.
one probably answer is that their so few examples of the weapon in the surviving remnants of Boddole Zer Main Fleet (Space War I) that reverse engineering of the technology was difficult, since from the TV series we know they had a few of those liquid plasma rifles.
the other reason is that all the automated factories which manufactured the weapons for the fleet did not have the "plans" for the weapons so did not produce this weapon for that zentradi fleet in that sector of the galaxy before they ceased operation.
Speculation is that the zentradi fleets have no mobile automated factories (such as the ones in all macross colony fleets) to produce advanced weaponry or weapon systems, although logical conclusion that the more simpler weapons were constructed and obviously maintained a sufficient production to armed new zentradi clone troops with basic weapons, ammo, armour suits and spare parts.
We can tell they aren't surprised by the weapon, which is why they probably ignored it, along with the zentradi space hulk.
the other reason why UN spacy do not employ plasma weaponry is another simply human reason, current kinetic weaponry, native or zentradi ("Impact" cannons) have been more then sufficient for all the current conflict between zentradi-zentradi encounters (Supervisor Army) and zentradi-human encounters.
really no need to go to the length to make them, although the fact that Michael sniper weaponry is effective, is probably proof that humans have not entirely given up on energy based weapons, as it looks more like linear electromagnetic-plasma. i.e. railgun, combining a kinetic slug with a plasma-shell (railgun use plasma to propel the solid slug down the barrel, hence electromagnetic barrier to contain the plasma)
just see the video below, you can see the plasma exiting the barrel behind the solid kinetic round.
quite beautiful if you ask me.
well, we now know that Bug armour is vulnerable to energy weaponry, even zentradi "impact" cannon rounds at point blank range cannot penetrate it.
it makes the kills in episode one by S.M.S even more impressive if you think about it.