To answer meddlers question about the snowball
"Have gameplay changes like those to gold bounties, scuttle, mage mana, resolve health contributed significantly? We didn’t see similar snowball increases looking at last year’s data and 2017 also had meaningful gameplay changes."
Ok so you didn't see similar increases last year? Let's see exactly what you did change this year.
New runes. 90% of the runes are scaling runes. So whoever is winning has the better rune advantage. Like this is simple shit whoever is higher level/more kills/more cs has better runes by default. Gathering storm is actually a healthy rune because it keeps everyone on an even playing field. Taste of blood/scorch/cheap shot/eyeball and many many more are unhealthy.
More damage because of the runes makes ganks even easier to pull off. Hence tying in with point 1 about immediately gaining a rune advanatge.
Take the tower because a whole wave of minions yourself and your trusty jg can easily shred it before he gets back to lane. Thanks for nerfing those btw that was exactly what they needed!
Now we have not forgot to press our advantage even more by taking the free 100g walking bag of lettuce before we take our rune advantage to mid lane. We might even get the tower before he respawns.
So now at 20 mins when baron spawns we can easily take it because it is garbage and they have no way of contesting but for some reason gives stats worth 10k gold and buffs that make it impossible for a team that is losing to have any shot at a comeback unless they pull an absolute brocolli play because the game is just that boring.
Look I'm not claiming to be an expert in balance. However your issue is pretty obvious, the new runes combined with towers being to easy to take and baron giving to much.
Champion balance is just a completely different subject altogether. Some champions can snowball harder than others and can combo runes to hyperscale. I'm not going to go into that though.