I don't think that is just one question you put in there
To me, an 'experienced writer' would be somebody that has, well, written a lot with going through all the steps from plotting, drafting, revising and editing. Maybe I'd say I'd start to consider somebody as experienced after they finish their third story? Like IMO the first story would be gathering experience, the second would be consolidating that experience (like some things could be a fluke and with the second one you can try it out again) and experimenting and from the third onwards, I'd say you have experience that you can fall back on where you know better what works for you and what doesn't.
I wouldn't say that the number of words is a good estimate for experience. Most people probably think of novelists when they hear 'writer' but somebody writing short stories would be just as much of a writer and I'd say they also start accumulating experience starting from the very first short story they write and I don't think they need as many words/far more stories to get the same 'level' of experience a novelist would need.
On that note: I actually think that you can be an experienced writer in some regards but not in others For example, I'd consider myself to be an experienced novelist but not an experienced short story writer simply because I have more experience with the former than the latter.
As for this part:
In general, I don't think there's a spectrum that has experience on one end and creativity/instinct on the other (I don't think that's what you meant just wanted to make clear how I see it as a sort of introduction to my explanation ). E.g. just from my own personal perspective, I'd consider myself as somebody who writes a lot by instinct. I almost never plot in the way that I sit down and think about what kind of characters I want to have or where the story is supposed to go, it just comes naturally to me and will fit together on its own. That's what I'd call my creativity/instinct. That instinct doesn't have that much to do with the experience I've gathered. I'm still as creative as I was when I attempted my first novel, I've just gained additional experience.
Now, for me personally, this experience helps in different stages. E.g. I know what to take note of while writing or doing the first edits of a chapter so that I won't have to clean up as much in the later stages of revision/editing. This isn't as much in regards to content (where creativity/instinct would be more important to me) but more in regards to how do I write something, where do I cut the chapters or change the perspective for the best effect. It also helps in revision where I can go through some things purely relying on my experience with previous novels to see what I need to take care of since I already know where my weaknesses are and what will likely need to be adjusted. (To the point where I'm currently sometimes writing stuff and then think 'ah, yeah, you'll need to adjust xyz when you revise this later' before I actually finish the novel.)
Well, not so much, I guess? It depends. I mean if we're talking feedback for something they've written, then sure, there might be something they could gain from that but I'd say that's the same with readers as well.
Then there's some universal advice that almost every writer has heard/read somewhere like the 'show, don't tell' or 'active/passive' things. I'm pretty sure that picking any random book about writing would tell them about the same. The only question would be: Does the person in question pick it up more easy/better when told by somebody they know a little/in a context where they can ask for more details/in a safe space - that kind of thing - or not? If it's easier in an environment like here on the forum, then I'd say they can gain more insight from the 'experienced writer'. (Then again, the guys writing the books on writing are 'experienced writers' themselves in most cases ... I think )
Other than that, I'm of the opinion that the experience everyone gathers and how it helps is very personal and specific to a person's own needs/preferences/writing habits. It's mostly something that is built with time on your own so there is IMO not that much a person can gain from the input of an 'experienced writer'. But then again I might just be the person that learns more (or maybe has learned more when I started out) from a book than the actual person