There's pretty much two ways you won't be overwhelmed: One is to learn about everything going on so that you won't be confused, but that will take time since there's so damn much in this game. Joining a guild and/or playing with friends will really help as you can ask others to fill you in on stuff you don't know about.
The other is just to stop caring and learn to accept that this game has way too damn much.. lol.

For starters, I'll point you to my overview post that covers most of the main features of the game. Hopefully that will clear up a few things.

I'd also disable a lot of the notifications in your game settings and chat windows such as marketplace items if you have trouble dealing with too many bells and whistles on your screen going off at the same time.

To answer a couple of your questions (read my linked post above first though!):

• The autopathing/quest marker can be pretty bad sometimes, don't expect it to be accurate 100%.
• Greeting is another way to increase Amity with NPCs aside from the conversation minigame. Amity is like "friend points" with an NPC and can get you access to special items in their shop or certain pieces of knowledge.
• Harvesting plants is part of the "gathering" lifeskill. Tools can be bought from NPCs, though they do not last long and cannot be repaired. Most people craft their own with workshops and their own materials or buy it off the marketplace.
• You can't talk to certain NPCs until you've fulfilled a requirement, and it'll usually say what it is. Usually you'll have to get to a certain point in the storyline quest, have enough Amity with a certain NPC, or have talked to the "head" of the town there, usually meaning the node manager.

You can get another 7 days free by using someone else's trial code for a total of 2 weeks free. Look for people that post them on this reddit - they can check your progress and if you hit level 30, they can buy you the game for $10 since they get an in-game pet for it, and you'll get the game free.
I'd also suggest staying away from the Steam version of the game, since you never know what might go wrong with Steam, e.g. Steam going down or having update issues. The launcher works fine and updates quickly.

Don't stress over not being able to unlock or know everything. It's simply not possible. I'm a hardcore human encyclopedia kind of person and I don't think I fully understood what was really going on until 1-2 weeks into the game. Just enjoy your stay and take your time.