I have some that get purple stems as well, no big deal really, I like to mist (foliar feeding) when in veg and just mix a bit of epsom salt in to be sure. You could probably google up pics of your strain and see if the pics also show that purpling to help determine if it is just a normal thing.
Generally you don't want dirt/medium to get on your leaves, but a bit won't really hurt, it can just cause mystery spots on the lower growth that will leave you scratching your head thinking something is wrong. Looks to me like the leaf tips may have gotten down into the mud, they stick there until the plant perks up and pulls them out. It won't hurt the plant to just cut those leaves off, since its your first grow might as well keep them looking pretty just for aesthetic purposes, makes the whole thing more enjoyable.
No recommendation on how to treat for pests. One downside of soil is bugs seem common. If you take your hand and disturb the top of the soil, do you see bugs fly out? Same when you open up the bag of soil. Thats what made me move over to coco, I just hated the damn gnats and whatnot that seem to always be present. You can do a top dressing, they sell stuff that is like a gravel you can put on top of the soil that creates sort of a shield to help kill off and prevent bugs, I have also seen just plain sand used. For bug sprays, look around here for some recommendations on organic products, doesn't seem like you have any crazy infestations or anything that would require the really toxic stuff...and well you probably don't want to smoke those products anyway. One general rule of thumb is to not spray the plants with pesticides once you have started the flower period. Given the two months or so for flower, most stuff works its way out, or at least doesn't get into the buds themselves, so by harvest you have a clean end result.
edit: well...that did sort of turn into a recommendation, but I am no expert so take it with a grain of salt.