How long can a plant be kept in preflower?

I believe they can stay in veg indefinitely with no harm. It's common to delay flipping the switch to fix problems. That's a big advantage of photoperiod plants over autos!
 
By "preflower" I assume you mean what is usually called "veg"--the time before you switch a photoperiod plant to 12/12. And yeah, sure you can top them in veg.
 
DominaMortem ! I actually vegged mine for about 12 weeks and its finished with first week of 12/12 cycle. And I had topped 2 times in veg.
1st at the 3rd node on initial stem. The topped again about 3weeks later, on initial topping and on new growth's 2 or 3rd node
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As she is now SCrog'd in 4sqft DIY box
 
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