I am sure for you at this point it is all so fascinating learning about this amazing plant. I have been reading all that I can and growing in earnest for about 2 years now and it still thrills me to open my cabinet doors and see how much my babies have grown.
I would suggest if you don't already, to get high times and or skunk magazines as there are many fascinating articles. I have read both Cervantez and Rosenthal's books and while they both are very informative, I have found many mistakes, discrepencies, and contradictions in their material. I would still recommend reading some of these books as there is great information there but it has to be taken with the mindset that these guys are not
the authorities on MJ, and their knowledge should be taken as part of a larger collective of opinions and facts of from many growers' experiences(including the wealth of experience here among our friends).
All that being said(sorry for going off on a tangent), I am still trying different techniques but I have found that there are 2 techniques that help maximise yeild. One is to create multiple tops(if the strain allows for that) by topping, or Fimming, or Bending the plants over either through LST or SC. The other thing is to make sure that light is able to penetrate down into the plant so that lower bud get to develop more. Some would argue that by cutting away the lower growth rather than opening the canopy will allow larger buds to develop on top of the plant and produce more. I haven't tested these side by side to see which is better.:ignore:
It is very difficult to say that one method is more productive than another method as there are many good methods that are specific to the individual grow; and you will find that there are some methods that you will master and have great success with while other methods will elude you. Hamster likes to top his plants to create multiple top colas. I absolutely love to FIM my plants to do the same thing.
To answer your first question...You can LST your plants that are already deep into flower so long as it is done gently so as to not stress the plants. You probably will not see a big difference in the yeild as the plant structure is already set, and the training methods are best used during veg for maximum effect. But it will still benefit the lower buds if your plants are very bushy as it will allow more light to get to them and cause them to develop more before the harvest.