Reranch may not be able to ship to all areas,if memory serves me right some states and borders will not allow products like nitro to shipped across.But I'm sure if you contact them they can let you know and maybe even give you a point in the right direction to a supplier in your area.
Florescent lights can cause UV fade or discolouration.But this does take a few years.ie:white guitars can take on a golden brown colour,although very light it is a slight change in colour which somem people don't like and some people dont really feel too concerned about.
You can spray one kind of laquer over another,but,you need proper curing time and there can be adhesion problems which will appear during sanding and polishing.Different clears have different patiena's which can result in a difference in the appearance.
Acrylic laquer's lay on top of each other in different levels,if you sand on an angle you will see each layer of clear.Nitro is self activating,as you lay each layer on it heats the layer below it slightly and melts into it.Some people use this to salvage older finishes.There is a product that cleans and reactivates laquer,but I can't remember what it's called,(sorry ).
For best results it is best to stay with a similar clear by the same company all the way through a spray.Similar products can vary slighty in formula from manufacterer to manufacterer.Most manufacterer's are more than happy to supply technical information on the product that includes composition,drying times and application.
Hope some of this helps.