you need to know the pH of the water you are putting on them... and water once a day seems a bit much... if they are drinking it that fast you might be root bound... a pic would be awesome so we can help you out a little more...
I trim off everything thats pretty much under the main canopy... especially in the areas that wont get much light... some ppl would say dont trim but imo i would def trim off those little popcorn buds to focus more of the plants energy in to the main colas
what type of lighting are you using? i noticed with my ladies that the bottom leaves or anything under the main canopy would turn yellow and die until i added some cfl's underneath as additional lighting to my 600w hps... helped with the yellowing but now i just cut/prune all that stuff...
yea i tried adding water before to cut down ppm and it made the problem worse... ended up flushing and saw a difference in new growth the very next day... imo id flush
yea if shes 2 weeks old she needs nutes... the yellowing could just be a simple lack of nitrogen which should improve... as far as starting flowering you could do that... ive seen some really good journals with ppl growing 12/12 from seed... im actually growing 4 that way right now just to see...
on of my first attempts at growing was done outside without great success... maybe bc im a home body type person so i prefer just having my plants in the next room... but ill see if i can help with a few of these anyway...
1) imo id wait 2 weeks max in a solo cup so they develop a decent root...
what kind of soil are you using? anything that already contains nutes? if so you just need to continue to water... i def agree with using the pots weight as a judge of whether or not to water... and as far as your ph goes it is def important and maybe this chart can help you some...