Hey, I found your post when I did a google search looking for similar info. Unfortunately I didn't find what I was looking for, but it looks like you've never been to a finals, so maybe I could help you - I went to both Toronto and Vancouver.
Will they be selling merch?
Yes, from my experience they usually have team jerseys, plushies, figures, posters, etc. They usually also have freebies like lanyards and wristbands.
Are there any other events that are alongside the finals?
They originally planed a fan day on Friday but canceled it. From the description they posted online it sounds like they may do something similar to what they did in Toronto, where they have places you can take pics in a 'champion select' screen, or on a faux caster desk. They also had posterboards and markers to make your own posters. Usually the devs are walking around chatting too. Vancouver didn't do this, but again from the description it sounds like they are in St. Louis.
Will there be any opportunity to meet any of the players.
Maybe. There wasn't one in Toronto. Vancouver did a 'pro meetup' thing where you could wait in a line and get your photos taken, however, the teams that did this weren't the teams playing that day. With St. Louis being on one day, I wouldn't anticipate getting to meet either of the finals teams. They'll probably be heavy into prep before the game and celebrating with the team after. Some times teams post on their twitter that they're going to a certain bar or whatever to do an unofficial meetup or hangout.
Also does anyone have any , other than the obvious dont be an ass, tips for the show?
Not really. Be kind to your neighbors, as you're going to packed in tiny stadium seating with them for hours on end. Otherwise, everyone is there with the same goal in mind - to watch LCS. Get loud, get hype, try to drown out the cheers of the other team with your own cheers.