Actually, STR_ZOMBIE_IMPLANT_ORIGINS is supposed to come before STR_OSIRON, as it serves as a gate to Osiron. But over the years more Osiron missions were added, and suddenly there are alternative routes to unlock Osiron.
Anyway, osironStakeoutSearch is a failsafe to ensure that if you have STR_ZOMBIE_IMPLANT_ORIGINS, but not STR_HUMAN_OSIRON_BOSS, then you get a high chance to roll this mission. If you have STR_HUMAN_OSIRON_BOSS researched, it uses the other, less common script osironStakeoutStandard.
The most likely route to get STR_OSIRON is through capturing a single goon from Surf Time, which has 22 % chance of spawning once you get surfer cover; after Promo III the chances decline.
Looking at my 3.5 game, I'm currently in May 1998 and I have had five Surf Time events already (months 7, 8, 10, 14, 16). I guess the RNG has been very bountiful for me. But even with normal RNG, you're very likely to get at least one Surf Time within a couple of months, 6 months at most. So if you play it right, you should have obtained STR_OSIRON by month 9-12 at the latest. Even if you don't do Surf Times or don't capture a lot of goons, you should have exhausted all the Secret Files topics around month 12 or so, which grants you STR_OSIRON. (Osiron Goons are actually an easy way to clean them up.)
Even if the initial idea has been to get to Osiron through the Zombie implant arc (after dealing with the Zombie MAGMA lab), etc. this is no longer the case at all.
Reducing the likelihood of Osiron Stakeouts through the actual primary path (STR_OSIRON) just delays the second phase of Osiron. The main goal of getting to the second phase of Osiron is guaranteed unlocking of the The Syndicate arc through Osiron Boss. This change essentially delays the syndicate arc a bit and makes it more RNG prone. Actually, as it stands, the best chance to get Osiron Boss appears to be to share the tritanium ammo with MAGMA, because then there is 25 % chance of getting tritanium ammo mission. which includes the boss. Apparently I should have shared the tritanium ammo tech to get this juicy mission. Doh.
I suppose you could either remove STR_OSIRON from Secret Files and reduce the chances of Surf Time considerably (to say, 5 % or less). This way unlocking STR_OSIRON would be much delayed unless you hit a RNG jackpot, like in your original concept. Or you could redesign how osiron is introduced somewhat or maybe even keep the zombie plot path as a secondary backup path. With this latter option you could also more clearly separate the first-phase Osiron missions (goons and fixers) and second-phase osiron missions (to get the boss to get to the syndicate).
