Don't forget that gifting units may also cause 'major breakthroughs' in scientific backwarded countries.
Don´t worry about that. It won´t happen in this scenario
Why not?
Well, the tech tree has a similar design to the "Raging Dragon" scenario.
Take a look at this:
* There´s only one tech that allow the production of 3 different Tanks: Mobile Warfare I
That tech allows the production of:
- M60 (USA)
- T-62 (USSR)
- Chieftain (NATO)
The trick is the tech that make them obsolete
In the case of the M60 that tech is: "NOT USA"
So, everybody have that tech except USA. Get the idea?
For example:
* USA sells a M60 to NATO. No technological breaktrough. The tech is the same
"MOBILE WARFARE I"
* USA sells a M60 to SEATO. It may lead to a tech breaktrough. SEATO may acquire Mobile Warfare I... but for what? they can´t do anything with that tech.
Why not? Because SEATO has these techs: "NOT USA"; "NOT USSR" and "NOT NATO"
Did I make myself clear?
The "major breakthroughs" you mentioned, could only happen with the nuclear weapons. In the case of selling a nuke submarine, a MRBM or a ICBM.