Well you are right about base clear being forgiving...to a certain point...if it was a metallic color, base clear would without a doubt would be the product of choice...if you make a mistake or just want to do a quick de-nib, all you would need to do is let the base coat flash, sand the area that you want to repair and re-base and then clear...runs? Cut and polish them out. With Single Stage metallic's...that can't be done and you never get the same gloss in Single Stage as you would with base clear.
If your Single Stage paint is totally cured and you prep the car fine enough, you should be able to base right over top of the Single Stage, even if you have a few small burn throws. Your primer underneath your Single Stage is catalyzed, your Single Stage is catalyzed so the only way you should have a problem with adhesion or wringing is if it wasn't prepped fine enough (wringing) or the Single Stage wasn't cured (wringing and adhesion)...I'm sure I don't need to say this but...to keep the car straight...always use a block wherever possible when prepping a car for that mirror like finish.
Best of Luck.