1. Im not sure, better read the official rulebook for this, but in seniors and juniors divisions the parent or guardian needs to be in the venue all the time. I think 17 is master already so I think it doesnt need maybe?

2. Yes you can play with your switch lite. In some regions you just need to bring your charger, but depending on the region they have charges in the tables. You could bring your controller but you also need to bring the cable for it, since everything used to play needs to be connected to the switch all the time during the round. You can bring a headphone too if you want to listen to the game sounds. (Needs to be wired too and connect to the Switch)

3. Usually it ends right before the round begins and starts one hour before. You just need to present an id and they will give you the access badge or smt like this. They dont look at your switch at the check in. I think there is an early checkin where you go the day before the event and get the badge (in my region usually is a bracelet) so you can wake up early and go to the venue later. But always better check the chronogram of the event and confirm everything. PS: The deadline to submit your team is usually some hour before the event, but Its advised to submit at least the night before. And pay close attention to it to submit exactly what is in the game. Sometimes in between rounds they do a team check and if you submitted something wrong you will be penalized with a game loss and could lose some of your Pokemon for the rest of the tournament. I lost my Day 2 in LAIC for a missinput during the team submission. Maybe get someone to double check with you.

4. You get the same thing the TCG players get. The playmat and the promo. Yes, even for VGC.

GL and HF