First disclaimer: nearly every player of any trading card game out there has pissed away a hilarious amount of money on said game. Especially adults with Real Jobs, but kids too.
I'd figure out what to do if your daughter is interested, but let her decide if she wants to do it and only pull the trigger if you get a solid yes. If she's not enthusiastic about it, it's definitely not worth it (in dollars or in dealing with thousands of cards).
Second disclaimer: I'm sure we're in a bubble right now for Pokémon anything (due to the success and popularity of Go). I checked eBay the other day, and the DS games HeartGold and SoulSilver (originally 34.99 games in 2010, remakes of Gold/Silver originally from 2000) are roughly $80 a pop at the moment.
I played the Pokémon TCG from about 1999 to 2001 (age 8 to 10), and Magic: the Gathering from 2003 to 2005 (age 12 to 14). They are very similar in a lot of ways on a high-level overview (both published by the same company for a while), with Magic being far more advanced/geared towards adults.
My grandma spent a stupid amount on Pokémon cards for me, and I spent a stupid amount on Magic (luckily quitting when I got my first job or else things could have really gone off the rails). No idea what my grandma spent on Pokémon (truly don't want to know, I know my parents weren't happy about that), but I was probably north of $1k on Magic.
Third disclaimer: I'm not familiar with the way the new stuff works, but going on how it was in the past and how Magic works, these are my answers to your questions:
1. My guess is that anything recent is compatible with each other, but older cards may not. There may be certain cutoffs where rule changes/etc make the different "eras" of cards not able to intermingle. Again, this is a guess, the truth could be a lot different. Look into this.
2. It looks like the three kinds of packs you're seeing are Trainer Kits (starter pack, basically), prebuilt decks (60 cards, ready to play), and booster packs (10? cards).
The Trainer Kit looks like it's two decks of 30 cards. This is an introduction to the game, with small decks to help learn (normal decks are 60 cards). It's probably the best way to learn the game, but the cards it comes with probably aren't great and wouldn't be all that useful for future decks.
Prebuilt decks are 60 cards put together in a playable deck with a particular theme or strategy. These might be OK, but generally are looked down upon by more experienced players (who all roll their own decks), and are not usually great value (may only have one or two "rare" cards).
Booster Packs are a random assortment of cards of particular rarity (for example: each pack might have 6 "common" cards, 3 "uncommon" cards, and 1 "rare"). These are where you get your "good" cards, but it's all a crapshoot and you have a pretty good chance of opening the pack and getting only 0-1 cards that are useful for you.
None of them are particularly great value, but typically once someone is established, all they will be buying are booster packs, as they will want to create their own decks.
The best way to handle this would be to have her play with a friend that already has cards, using decks the friend has already made. Then she'd figure out what she likes and buy her own cards to make her own decks. That's of course not possible if you don't already know anyone into it.
3. Those bulk packs could be OK, but they could also be total junk. If you don't know what you're looking at, it's probably best to pass on those. If you knew someone that knew what they were looking at, you could enlist their help.
4. The "collectible" side and the "playing the game" side are rather different. Street value of anything in any preconstructed deck of any kind is basically zero, so if you care about collectibility, you'd only be interested in the booster packs. If she's not into the collectible side of things, there will likely be valuable cards that aren't useful for her particular deck, so she could trade those for equivalent valuable cards that are useful to her.