The rule of thumb I take (others may disagree), is that 2-pack can be OK over a finish that was iso-cyanate based originally. I have got away with '2-packing' directly over the original colour if nicely flatted and absolutely sound / grease / polish free.
I've never had the guts to try and put 2-pack directly over an old suspect / cellulose finish - its chemically quite aggressive and the risk of pickling always seemed too great to take the chance; (the work / delays in completely sanding off a 'reacted' car are too 'orrible to contemplate!).
Once the car is prepared and ready, the effort in applying a single barrier coat is, by comparison, negligable and cheap. Only one uniform coat is needed (don't rub it down unless you get runs - and if you do, re-coat that area.)
In my experience BarCote is almost bullet-proof against paint reaction .
You are quite right about T-Cut - works great on cellulose but 2-pack finishes are VERY hard (thats why they hold up so well), and T-Cut doesn't really touch it. There are other , better materials designed for the job, (e.g. ask you're paint shop if they have the Farecla G3 compounds), but it still takes effort - thats where the air-mop comes in!
The trick with 2-pack laquer is to get a good finish straight out of the gun - which is MUCH easier than with the old 'drying' paint systems (they shrink), whereas 2-pack cures chemically, flows better and minimises that orange-peely effect so common with cellulose.....it is less prone to runs (within limits!).
My only problem with this type of finish is that it can look 'too good' on the wrong type of car......always felt that (say) a '68 MGB looks 'wrong' in 2pack but dead right in cellulose.... entirely different sort of finish.
My Cobra (pic I posted a few weeks back) was finished with 16 coats of cellulose over a 6 month period, with each and every coat carefully flatted with 1000 grit. It took a lot of prizes because of the paint finish which looked so deep and rich compared to the 2-pack finished cars......even though they were very shiny.
(Took a bl**dy lot of work keeping it looking like that though! - which is where modern finishes win out).