Hopefully Vickie will answer in more depth
This is what usually happens
I have done many tours but have never been able to book in online, JY do a group booking that’s the reason why, some people who upgrade on a flight can book in online..
Yes you have to put your luggage on the scales to be weighed, just make sure your not over the limited for that tour as it can be expensive to pay of excess baggage.
Your seat will be allocated so no you cannot pick a specific seat...I usually ask when I book for a aisle seat, or when I get to check in, sometimes it works other times it doesn’t.
Have a great tour and leave a review when you get back..
Cindy