1. For a party you need to check Faction and World Chats, everyone is posting stuff like "F8 1/4" or "DB 1/4", people don't use the region chat on F8 anymore to look for parties, Cerulean Chat is more active than Crimson so that can help.
2. Earthen Coins were from the last event so you can't get those anymore, about Sterling Crystals, there are several options, you can get 2 from the NPC Gillyu in the Northern Continent (if you don't know where that is, complete the story quest). On Mushin Tower, the NPC Trader Gang Sanhae, will sell you 3 pieces, right now there is the soul league event, if you manage to get to gold level or higher, you can buy 4 from the NPC Moon Hojoon in Solak and lastly you can run old dungeons in hard mode to get those as well (but this never happen only get the gear and friends that can help you).
3. Stage 5 is available to anyone from the very beginning but I'm assuming maybe ppl don't take you there because of your gear, is a good idea to focus on completing soul boost so you can get the Starcross Equipment with that you can start doing stage 5, also try to watch youtube videos to learn the mechanics of the new dungeons that will help, because right now there's a lot of toxic ppl that won't help or don't like to have ppl that doesn't know all the mechanics of the new dungeons.
4. I can't answer that, I don't know anything about it.
Focus on doing Soul Boost, to get the basic gear you will need.
Hope that helps.