Hasan Othman, DDS, MS, PhD

there should be a number on the tray on the occlusal surface in the molars area .   If you can’t read it on both I would guess the newer should be more visible.  

I would try and keep the area clean and may be use peroxyl mouth wash for now until the space is completely closed.  The frenum is usually removed (Frenectomy) after full space closure and this will help in the stability and keeping the space closed.   

If you have a healthy bone and periodontal tissue and you have no contraindication for  extraction treatment, I do agree with your orthodontist.  extraction will correct your midline and improve your overjet (overbite) and give you a better overall results.  Just a reminder it's hard for anyone to give you a definite answer without full records and clinical exam.  Good luck 

I'm not sure why brushing after a meal will erode your teeth, unless your meals always contain Soda or acidic food.  I would try and use a soft tooth brush with gentle brushing followed by a mouth wash.  You can always do a very thorough oral hygiene at night before you sleep (brushing, flossing, and mouthwash).

Back to your main question.  leaving you aligners out of your mouth for 2 hours or less per day should not compromise your treatment. Discuss this with your dentist to be sure.   

it depends on your bite and the rest of the teeth.  It's not possible to answer from one picture.  looking at the crowding on the lower alone, most probably yes it can be fixed with braces or invisalign if your gum and teeth are healthy, but you also have to consider your upper teeth and bite when deciding on a treatment plan.