Hey! I think it's great that she knows two tricks already, she will learn and get the hang of it. I would practice with her every day for 5-10 minutes at a time using a yummy treat that she loves. It just takes practice and patience but they are smart little things! I don't think there's really a "guideline" per say with their age and what they're supposed to know, it all depends on what YOU teach them. My mom has a 4 year old dog and all she knows how to do it sit because nobody ever taught the poor thing anything else, lol.
Bathroom is not a bad place to keep her. If you don't have room for an x-pen or crate, that's your safest bet. I wouldn't worry about the light thing. Dogs eyes adjust. You won't hurt her eyes
Whether it be dark or too light, it's no big deal.
I fed Jackson dry food mixed in with wet food for a while but wanted to get him out of the habit. I just free-feed and leave out dry kibble all day so he can go over and munch/drink whenever he wants. I will mix in wet food every other day or so as a treat for him. He's 5 months old.
Bad breath... haven't experienced that yet. Jackson's has never been bad, really, unless he ate something bad, lol. I've heard PetzLife products work wonders.
PetzLife Products Inc - Products For a Healthier Pet I've never brushed any dogs teeth but I may start with Jackson just to get in good habits.
The weather thing.. each dog is different, really. Jackson doesn't mind the cold at all. He played in the snow for an hour at a time with us, no clothes or anything on him. He's bigger though weighing in at nearly 10 pounds at 5 months old so maybe that's why the cold doesn't bother him. But I have outdoor trained him since I brought him home at 9 weeks in December and he's been fine.
Housebreaking, I basically just kept Jackson on a tight schedule for the first few months and tried not to even allow an accident to happen by taking him out every half hour - hour that I was home. When I wasn't home, he went into his crate, where he never once had an accident. He's moved on to the x-pen when I'm not home, but mostly now I just leave him in my room and he's absolutely fine. Consistency and schedule is key.
I'm sure others will give you loads of other better advice!