1-What controls the shifts in a 4L60E 4L80E and can I replace the PCM with a vacuum modulator.

The answer is the PCM is in complete control of every aspect of when and how the 4L60E shifts, Line Pressure and Lockup. If the command is given barring any mechanical failure clutches, Band ETC the transmission must make the shift or slip and burn itself up trying. Alternately It cannot shift before its commanded.
Lockup again is strictly a function of the PCM turning on the lockup solenoid and pulsing the PWM solenoid.

I hear a lot of people say my converter locks up sometimes and other not what wrong with it. Well If your converter locks up properly anytime. Then its more than likely a tuning issue and not a TRANSMISSION/CONVERTER issue.
On the matter of the vacuum modulator, It cannot replace the PCM it only takes over the PCM ability to control line pressure’s. I have a lot of people ask if I do the "vac mod" will it fix my shift timing issue? The answer is no.

2-Shifts become softer or slipping after a converter install.

No in fact the shifts remain the same as before the converter install. They feel softer because of the converters shift extension/looseness. The transmission it self is doing exactly the same as it did before the converter. Now It can be advantageous to firm the shifts up since more torque is being transferred though the trans at WOT.

3-Increasing line pressure in tuning is a good way to firm up shifts.

Well it will firm the shifts but at a price of more strain on the hard parts and greater load on the pump. A common failure for instance to raising the line excessively can be the failure of the input drum at the 3-4 clutch snap ring or even pump failure due to the extra stress. There is even the possibility of having so much line that there are enough cross leaks to partially apply clutches or bands when they should not be on. In my opinion the proper way to improve shift quality is to install and shift kit. I have no preference to type since all I have seen with only minor variations accomplish the goal not so much by raising line but by increasing the rate or volume of fluid being supplied to the clutch in a given time.

4-Synthetic fluid will make your transmission slip.

I will likely get some flack on this one but this has not been my experience at least not with fluids that meet or exceed the requirements for that particular application. What this means is the product regardless of type or brand must meet the minimum requirements it specifies. For instance if it says meets or exceeds dexron/mercron requirements then it must perform equal to or better than the specifications of that fluid. Now don’t think I mean that everyone should go buy synthetic because personally I think its overpriced for what you get. The only real advantage I have seen is the ability to with stand heat better than petroleum based products. However if you have adequate cooling this should not be and issue anyway, IMO use the fluid you like or what your converter or trans manufacturer requires. We have no requirement here.

5- TRANS Fluid can get to cold and gel at subzero temps.

No it can’t at least not at any temperature where people can live. Think of it this way if it could then in Canada or Alaska the fluid would be gel in the pan in the morning on start up and would wipe the pump out immediately. I have poured fluid from a bottle at -5 degrees and while it’s just a little thicker it’s certainly not gel. Transmission fluid is made to have a very stable viscosity at all temperatures. I am sure like all liquids there is a temp where this could occur but none I have seen.

6-Allowing a transmission to set either in or out of the car for extended periods (months/years) can result in failure.

Yes actually it can. As the transmission sets for long periods of time fluid will slowly run down and away from the clutches and hard parts. The seals can even dry rot similar to the way tires do and more in damp climates moisture can enter and cause rust on hard parts and even under the lining of the friction materials and cause them to separate. I have heard the “It worked great when I took it out 2 years ago story” and its likely a true statement only to hear later that someone put it back in and it died in days or weeks or slipped or other wise acted up from sticky valves. My suggestion for storing a transmission for any length of time is to plug all the holes and fill till the fluid begins coming out of the overflow tube. Then cap this to as that will insure all parts are submerged in fluid. You will just need to drain before restarting after storage. This may help avoid some post I see with transmission complaints after taking a car out of storage that has been setting all winter.