Some questions about Pokemon games from greenhorn amateur

I'll answer based around Sword and Shield.

1. You can trade between games of the current generation (8 for Sword/Shield). In order to move Pokemon from a different generation, you'll have to transfer it. Transferring is a 1 way process, so you'll move the Pokemon from one game to another. I assume that for moving Pokemon from Sun/Moon/Ultra Sun/Ultra moon will use the Pokemon Bank service as has been the case before. Though you'd find it cheaper to just trade with someone that already has the Pokemon you want.
2. Based on the previous games, there won't be any gameplay differences between the 2. There will be at most, only minor differences in the plot, not enough to justify buying both for that alone IMO.
3. In terms of Sword/Shield, you'll only need to get 1 unless you want to pay for 2 games. XD
4. Don't get into the competitive gameplay if you just want to have fun. XD Feel free to reply if you want to ask me something else (or for a better explanation).

For the Pokemon Refresh, I don't think they'll remove it as Eevee needs that mechanic for one of it's evolutions, but they have done it before... As for the rest of those, they've been in the main games for so long (hold items were released in Generation 2 (1999) and abilities/natures were introduced in generation 3 - (2002)), I would be massively astonished if they were removed! =P

To expand a bit on the trading, there's also a built-in online trading service called the "Global Trade System" or GTS that's basically Have/Want matchmaking. You can put up your pokemon and ask for ones from the other games in return. You can also search for any pokemon and see what other people are asking for. When a match gets made, Nintendo's servers make the swap and send out the pokemon the next time you connect.

It's perfectly possible to enjoy any game in the series by itself. The main reason why you would buy a second game in the same pair is if you personally want to catch the version exclusive pokemon instead of trading for them. The second releases of the generation are inconsistent - sometimes it's a remake of an older game (Fire Red/Leaf Green, Heart Gold/Soul Silver, Omega Ruby/Alpha Sapphire), sometimes it's a sequel (Black 2/White 2), sometimes it's a remaster of the previous pair (Yellow, Crystal, Emerald, Ultra Sun/Moon). You generally won't miss much if you skip them, but there are more significant changes than you'd see between two games of the same pair.

As far as prices go, Pokemon games tend to hold between 50% and 100% of their value in the long term. You can usually save some money if you wait to buy when the next game comes out, but even the first game boy games are $15-20 today (MSRP was $29.95)

i only played the 1st two generations and loved it. played gen3 recently on my mobile (emulated) and the game has improved
watching the gameplay of modern games had me ordering a 3DS to play the rest of them.
although i currently like the game for its gamplay and less for pokemon (cant catch all 666 pokemons). exploring the new evolutions and moves is fun.
i would recommend to buy only a single game as they offer no change in gameplay. and as stated before you can always trade for missing pokemon. (we had it hard with the gameboy connections -_-)

Play Emerald, Platinum and Heartgold/Soulsilver with an emulator.

You only need one game of a generation if you want to play for fun. Only collectors buy every pokémon game.
If you know another player that have the game, you can trade with them. Also it's the only way to complete your pokedex.
Pokémons that you get from trade have a boost in xp. This is a plus for training.

Every game have it's own legendary pokémon. You can choose the game that have the one that you like.

better play fangames like fireash https://fire-ash.webnode.com/

As a Pokémon greenhorn, maybe Let's Go, Pikachu/Eevee would be just right for you since it's kind of designed for beginners who know Pokémon Go at most. You certainly can catch Dratini there. :)

I love Pokemon, I still remember how many times I played Pokemon Blue on my SNES with the Super Game Boy, and Gold was even more beautiful! I still love to play Emerald and Leaf Green on the GBA, unluckily I played only few times the DS ones and I don't know much about the new games but I still think it's an awesome franchise, it's so great to play them =)
So thanks from me too since I don't know anything about the new games, but I'd love to play them in the future!

You don't need to buy multiple games to have fun / get all the pokemon you want - with online trading it is fairly easy to get pokemon that aren't available in your game these days.

There are various methods for trading pokemon between generations. Generation three games and up (including the remakes and/or virtual console versions of the generation one and two games) are all 'compatible' although you may need older hardware to do it.

Just having a pokemon and having a competitive pokemon are entirely different things though. Almost any pokemon can be competitive as long as it has the right attributes. You will just need the right moves (and certain moves for certain pokemon are only available in certain games so that will likely require switching pokemon between games and consoles), the right IVs and nature (which can usually only come from a breeding program that will likely benefit from certain features in certain games) and the right EVs (which come from training your pokemon likely through battling and again may benefit from the pokemon being in specific games).

This is why I have 2 x GBA, a gamecube, 2 x DS, 2 x 3DS and every pokemon game. For the love of God don't get started with this bullshit...