yes yes and yes
Trimming- a lot of people clean up the lower 1/3-1/2 of the plants after they have been through at least 2 weeks in flower, or sometimes before going into flower, cutting off these "sucker buds" helps the upper buds/plant, since they are tiny and dont get much light they will never really pack on weight, youll end up with just popcorn buds, so instead of allowing energy to go to these tiny, like 0.1g, buds a lot of us trim them off so only what gets light stays, so that upper 1/2 in turn gets all that energy that would otherwise be going to those tiny, kinda usless buds...
Clones- most people will take clones before going into flower, they are taken during the pre-flower trimming. the hormones in the plant required for rooting are the veg hormones, so after a few weeks in flower, when the plant stop stretching, that is the time when the flowering hormones outweigh the veg hormones, thus allowing bud development rather then just pure growth like we see in veg. now because of this hormone imbalance it will cause any clones taken after the first 2 weeks of flower to have to re-balance out the vegging hormone in order to start rooting, which takes much longer, so if you didnt take any clones yet, and they are still within the first 2 weeks of flower then yes take clones and get them started, if they are past 2 weeks in flower, take lower branches that wont amount to much at harvest while you are trimming the plants up a bit... use those trimmings as clones but remember they will take like maybe 3+ weeks to root... where as personally i usually have to wait about 10 days to be rooted well.
cleaning up the lower half does both, it does help with air circulation and light, as stated above those lower branches wont really amount to much of anything other then wasted time trimming and curing for a measly fraction of a gram at harvest... so what ever lower branches on the lower half of the plant that dont get much light because they are well under the canopy... main branches and side branches from the main branches, anything that wont amount to much just chop off,...
fumbling around grow journals im sure you can find many examples of trimming up plants for flower/after the stretch has ended.