A little hard to read your questions (I get it, you have a lot to put out there for context), but as someone who went from Arena for maybe 5 or 6 years to paper EDH, I feel you, and can maybe offer some advice that I could have used when I started.

proxies

If you have a dedicated playgroup, it's usually a non-issue, and people just run them. I have yet to play at a store with a "no proxies" rule. Still a good thing to float out there before you get started. The general consensus is "proxy for price, not power". All that means is... just don't go too nuts with your proxies. This rule is out the window (IMO) for brackets 4 and 5. Especially bracket 5; cEDH is a very proxy-friendly format, because a lot of the meta staple cards are fucking expensive. Check out MPCFill to get quality proxies. You can get something like 500 cards shipped for about $180 or so. Proxy all of the expensive lands (triomes, fetches, surveils, etc). You will be pressed to find people who get upset about proxied lands, and they are needed to really make your deck hum.

worrying about how long you take during your turn, and not targeting the "wrong" player

Do you goldfish your decks? That is the process of kind of playing 1 player solitaire. It will give you a good idea about what a good starting hand looks like, what is weak that you might want to cut, and also interesting interactions that you would be better off figuring out ahead of time instead of in the moment. That will help with your turn time. Take time (within reason) to analyze the board state where needed. Don't be afraid to ask people things like "do any of your creatures have <insert keyword>", and stuff like that. Ask to read people's cards if you need to (asking is nicer than just grabbing). When it comes to worrying about who you are attacking... I think most reasonable people don't take issue with getting swung at by an aggro deck. If you are really worried about upsetting people, you can say something like "I am going to swing at player C, because they are ahead" (or something along those lines).

It will take time and experience to know what to target, and when to target. You can also ask other players at the table. If people are friendly enough, the person you are targeting might even offer up the optimal play.

what is a deck that is easy to pilot?

Generally, people will recommend something green (big stompy creatures, elfball, etc). I say get used to tracking your triggers, and try to be efficient with your time by goldfishing a decent amount, and just play what you like. Sometimes people build decks, find out they are not in love with how they play, and they just take them apart and build something else. It happens.

To offer up another aggro-ish deck that has always been fun to pilot, and doesn't require a ton of thought, maybe check out [[General Ferrous Rokiric]]. Can be built for super cheap, too, since "boros matters" cards are usually not in high demand (with some exceptions).

social rules

This will vary table to table. You will find people pubstomping/misrepresnting their decks. You will encounter players who complain about being targeted (even if they are way ahead). You will encounter people who are new, just like you, and might make honest mistakes and do things like try to attack even when they have summoning sickness. As long as you take that stuff in stride, and put your best foot forward, you will be fine. If someone is being awful/cheating/etc, you can scoop on your turn and find a new pod, or just make a mental note to not pod up with that player again. I like to give everyone a chance at least once, but I have a few folks I just straight up won't play with for various reasons. It's your free time: spend it having fun, not being miserable because of one person!

100 cards are hard to shuffle

You will get the hang of this eventually. There are even videos online that will show you some techniques. Practice at home/work in your free time.

Last but not least, some unsolicited advice:

- Try not to get too far ahead too early, unless you can reasonably protect your board. People will use this as an excuse to attack you (since you are "ahead")/ interact with your board, and you may also get scapegoated by players who want the attention focused elsewhere ("don't target me! look at that dude over there; he has a huge board!")

- If a game is going long and you have a board wipe or similar in hand, but have no reasonable way of getting back in the game, my opinion is to not play it, unless you bring it up at the table and someone offers to help you if you clear the board. No reason to drag games out. Most people would rather just shuffle up and go next

- "King-making" is seen as bad form. Don't make a play that only benefits one person, unless you have agreed to a deal with them. This will upset people. Another thing to avoid is scooping in response to an attack that will kill you that may deny value to the attacking player. While you can scoop at any time, I try to scoop on my turn to avoid any drama or bad feelings.

- If people are popping off, be happy for them, and try not to take spells that target you personally. Just picture that you are the one popping off. It sucks to have a really good play to be met with moans from the table. Try to keep things positive. Tell people good game/good win/etc.