General

For no. 4 The game isn't pay-to-win. It's more like pay-for-a-shortcut. You can get far without spending a single dime, it's just a lot longer than someone that would spend NX.
Spell Traces and Star Force has made it easier for unfunded people to do decent damage.
The game isn't dead but the population is spread pretty thin due to the sheer amount of training maps. There's a lot of variety for grinding, a lot just gets overlooked.
If you make a new character, do all of the theme dungeons and story quests. You should get decent gear from doing them for someone starting brand new, and they're a good foundation to build upon.
Jeweling is a great profession for getting items with potential.

That's just some advice I have.

Idk about being expensive, in the case of thieves they're actually the best because of the ridiculous %multipliers on their main skills. The only reason Shadowers become prohibitively expensive is when you can basically cap on any other thief class with that gear, at which point the speed of midnight carnival and meso explosion added on with the fact that they can hit double the cap damage turns it into far and away the strongest class. Of course, besides that case, they are probably the most cost efficient of the thieves.

1) I understand something about items turning into a scroll if it booms....?
-Enhancing with star force past 12 stars has a chance to explode. If it explodes, a trace of your item is remaining (potentials / scrollings, no stars) so you can apply it on a "mold" of the weapon (another clean one basically). It only works with star force. Tyrants experience it much earlier (I believe the 6th star can begin to explode) and cost much more.
2) is the alchemy/ forging/ jobs thing still relevant?
-Smithing is huge cause you can craft cubes, alchemys useful cause you can craft boss / PDR pots. They're relevant, but rather time consuming. You need to maintain your rank of master/meister when you reach there by constantly crafting every day. You will decay if you don't.
3) tips on how I can catch up (is it worth trying to catch up)
-You'll have alot to catch up. I can't tell you if it's worth it. It took me a lot of time and a lot of friends to get to where I am now, and many people never make it past 300k range. While 300k range isn't horrible, it pretty much doesnt do anything for you in this game.
4) is it still pay2win
-Yes. However, it's possible to get to end-game without paying. It takes a lot of time and connections. Even with these two pieces though it's not guaranteed you'll make it without spending some money.
5) anything else I should know/ advice/ links where I can hunt stuff down ( I know how to search still but I don't know where to start)
-There's a decent level up guide online I believe. Links: You need Kanna,DA,Xenon for sure. These three are the biggest Damage raisers. Zeros good, Cannoneers good, Phants good. Lumi and DS are more for bossing. Kaisers useful for HP and BT's good but no creation option.
6) for Shadowers is it still ridiculously expensive to get geared up and going in khaini and are D.Khanjars like catching unicorns still
-Every class is ridiculously expensive to get geared up. Some classes just have less of an expense to cap. As a comparison: (Capping on a DK might take 200-300b while capping on a NW might take around 400-500b) random numbers tbh I have no idea. In terms of gear though, Shadowers are thieves so yes they're expensive. D.khans still sell for a decent price. They're rare in my world from what I've seen.
7) are 50% scrolls/ Chaos anything- relevant for selling? I'm meso poor but had decent sellables for when I did play.
-Best things to farm are elite bosses which give epic pot scrolls, clean slate 10%s, and innocence scrolls. Those are probably the best scrolls to sell. Regular Chaos scrolls are pretty bad since Chaos scrolls of Goodness and Incredible Chaos scrolls of goodness exist (don't lower your stats).

Is it worth playing? I don't know. I play because I've made it I guess. Even now though, it's just weekly doing the end game bosses and then looking for ways to improve my range and selling stuff. I spend maybe 2-3 hours at most a day if I really need to on it. Other than that, I've been playing other games.

I came back after a 4 year break too just for the memories. Alot has changed and at first i regreted it due to how much the game has died. But if u can find good people its not too bad.
Items turn into traces if they boom only with starforce
Yes
Itl take alot to catch up
It is still pay2win
Id say its gone worse for shads, be prepared to sell some organs
Not so much

Shads are very expensive. I have like 0 funds on mine and im at 40k range and thats buffed at level 160. However i still find my shad fun to play