You can play with altering THC content in the ways you mentioned, but it is only going to sway it 10-30 percent at most. When we are talking about 0.2% that is not very much loss, and your CBD % will be affected by all of those methods. Most people pull early if they are trying to stay under for THC.
I have had similar ideas though, about spreading out flower under UV light or heat to degrade the THC. At the end of the day, it isn’t worth it; you should just find a cultivar that will test legal when grown and treated right. There are some CBD varieties compliant for smoked flower that are emerging (Soverign Fields claims a 13% CBD, <0.01THC).
Hemp is mostly grown from seed, due to the sheer scale. To grow for legal flower for CBD, @Tommy_McCain is correct that the best way to do it is find a clone with a good ratio (1:50 or greater), so that you can take it to full term and not go over THC %. This is exactly the reason CBG is becoming a huge thing for smoked flower next year, as there are CBG cultivars that will test legal when grown from seed. (Please see my thread here.)
from Potter, D. J. (2013). A review of the cultivation and processing of cannabis (Cannabis sativaL.) for production of prescription medicines in the UK. Drug Testing and Analysis, 6(1-2), 31–38