You said you have played the game before, I was wondering why you haven't figured it out yourself some of the answers to your questions 🤔
Anyway here are some answers in my opinion:
1. How you use saved souls is up to you. If you're starting a new character, you would like to have unique items as much as possible so you can use it when you reach higher levels and in arena. Then it's worth it to lose all in the shop for a chance to be stronger. The souls will be replenished by gifts & etc. so it's justifiable. Using 24hr stamina is good but if you're new and not buying souls then save them for a chance in the shop and let the game run it's course.
2. Wings can only be acquired in the shop, either 10 or 100 souls. Nowhere else.
3. Percentage of drops, combine & etc. is very low if you're talking about unique items, so lower your expectation in acquiring such items and be patient.
I don't know any site or fanpage for darksword that you can view for guides or powerbuild etc. I mainly read the old forum from their old site to look for answers but it's gone now and replaced. You can visit their new website (Nanoo) and look for guides if there is one. Unfortunately there are no more tabs you can use now for general discussion, items, characters, troubleshooting and the like so you have to create a topic for each of your inquiry.