I can't answer all questions in full detail because I've never designed a keyset myself but I hope I can point you in the right direction.
1) Anywhere from nothing to hundreds of dollars. Most IC expenses come from renders. If you make the renders yourself they cost nothing, but if you're not comfortable with doing that you can commission someone else.
2) This is the way I'd do it: Figure out what colorway/design/inspiration I want to base my keyset off of > decide on ABS vs PBT and doubleshot vs dyesub depending on my design and colors > find the exact manufacturer I want > tweak design/colors based on manufacturer's production limitations and/or stock color choices > post IC looking for feedback > contact vendors, they will speak to the manufacturer. Your vendors should be able to help you figure out the rest.
"Pre-IC" is a bit of a meme IMO. Anything you do prior to an IC shouldn't involve making a public Geekhack post, just talk to your keyboard buddies or reach out to some keyset designers for some feedback. Not subjective stuff like "is this shade of red good" but asking about kitting, vendor choices, the consistency of the keyset etc. There's been some "pre-ICs" posted and it's usually super underdeveloped stuff. In 2023 it's pretty much expected to show the final product when posting the IC. The community gives feedback but usually it's only small things that need to be corrected.
3) You can learn it yourself or reach out to someone with a bit more experience and commission them to make renders for you. The latter will cost you money, though.
4) Make sure you follow your manufacturer's guidelines.
If you wanted to run a GMK keyset, you would look at this and design your novelties according to their limitations/specifications.5) I have no reason to believe your vendor (especially lead vendor) would refuse to help you with your keyset. Maybe not stuff like picking colors but rather making some decisions about MOQ, number of child kits, inclusion of less popular layouts etc. It's in the vendor's interest to make the keyset as good as possible because they make more money if the keyset does well.
6) You'll appoint one vendor to be the lead vendor and they'll do all the communication with the manufacturer and be your proxy for color samples and/or prototypes if needed. Some vendors like working some specific regional proxies but you should be able to pick and choose which vendor you want for each region. Make sure to ask the people which vendor they'd like to see proxy the keyset for their region. Unfortunately there's some vendors out there that don't do a good job so it would be best to avoid those.
7) Instead of starting at 0, enjoy the hobby for a bit and you'll learn lots along the way. Watch content creators, chat with some people on Discord, check out other threads on Geekhack. Especially check out other keycap ICs and GBs here on Geekhack and look at the renders, look at the kitting, look at everything and compare it to other keysets you've seen. What do you like or dislike about some keysets, why is that? You can learn tons just by participating in the community for a few months while just having a good time instead of joining and immediately being overwhelmed about your keyset.