I'd been thinking about the arena issue! And I don't think subtracting tickets from the bottom sections is the worst idea, but I had some other ideas on potential solutions to deal with some of it.

1. Bot Matches
Just like, guaranteed bot matches. You purposefully go against only bots with no chance for going against human players. This way, you'd have an easy space to learn the ropes without people who are good at the game flattening you, as well as if you afk, you're not affecting any other players. Because the big complaint I see is that people doing afk ticket farming makes it not fun for people who want to actually play, so if they could just fight bots instead, that would solve a big chunk of that.

2. Mini Matches/Shorter Matches
I know a part of the whole system is “Go ten rounds and try to get strong stuff so you can beat everybody”, but maybe we could also have shorter matches that are just one on one (maybe you get ten creatures to pick from and can use up to five, per the usual team amount), or maybe up to three rounds (still buffed creature amounts). Personally, I got turned off the whole system by having to queue up, and then I got in tenth because I had no idea what was happening, and then had to wait for bots and wandered off and never went back. Shorter matches would help people learn the ropes with less time commitment! You would also get fewer tickets due to shorter, presumably easier matches, and can also just do versus bots in this option.

3. Maybe adjust it some based on participation?
I know people were already talking about lack of participation = kick, but it's also been mentioned some matches were almost entirely afk players. So maybe rather than subtracting rewards for ninth and tenth, and leaving the other afk players to get things, if a player hasn't bought/placed any creatures, they don't receive any rewards. Plain and simple. So you have to do at least a little bit of something in order to get any tickets, but it doesn't matter where you place.

4. Reputation system and/or temp bans from User Matches
In Pokemon Unite, things like being afk or getting reported or quitting out of matches/getting disconnected would lower your reputation. (I think that's what it was called.) When this was low enough, users could only play matches against bots (and with friends in friend mode), therefore generally preventing those people from causing problems in matches full of real people. So something like if you get kicked from a match, you lose a few points of this, and when it gets low enough, you're forced to do bot matches exclusively until your behavior is good enough that your rep gets back up high. Brief timers (never more than an hour, I'd think) would also discourage afk play.

Keep in mind I haven't played the arena more than once, so I don't know precisely how well all of this fits, but it was some ideas that were bouncing around in my head. With a little workshopping, people can probably make them better and more precisely suited! And hopefully Arena gets more tweaks to make it better.

OP's suggestions seem reasonable; saying this as someone who plays Arena at least once a day (has become a morning ritual) and used to play much more often. Honestly the current method isn't a major deterrent for a clever AFKer anyway--all you have to do is play long enough for two bots to drop to 0, which doesn't usually take that long. Having the reward penalty based on lack of activity prior to dropping to 0 makes more sense.

I also like @Orgetzu 's ideas, but assume they would be much more challenging to code.

I think it would also be nice to add rewards that aren't just based on how you place at the end: Ex, extra pebbles/tickets for how many 3-stars you have in active play at the end. Extra rewards or some kind of achievement system for ending the game with less popular synergies unlocked or with certain themed teams (such as having all the enderlings or all wolves) would also be interesting, and would be something to do for experienced players who are tired of racing to max tactician over and over.