1) i realized i was getting better when songs that i used to listen to when i first started playing and used to be like, "man, how the heck is he/she doin that?"...i would find them again much later on and pick up my guitar and be like, "oh, this is what they did, well that's fairly simple."
people also let you know you're getting better.
2) don't know if i've found my tone. i play through (as you know D) a semi hollow, so thats the majority of whatever tone i use right there. my favorite setting is Bridge/Neck combo with the neck turned slightly higher volume and tonewise.
3) i don't know if learning will get easier, because it'll take the same discipline that it took to learn the pentatonic scale earlier on, to also learn more difficult stuff as you try more advanced guitar playing. i think the feeling of getting easier is largely the ability to not be frustrated as much when learning harder stuff because you know you can do it after coming so far.
my 2 pesos