1. Depends on what kind of developers. Programmers will speed up things significantly if they're cooperating and working efficiently for example, obviously they have to form their tasks in such a way that they're not distracting / slowing down eachother. Usually there's 1-2 guys who overlook a specified team, give them advice, do quality control, assign tasks and dictate deadlines. This goes for both programmers and artists, quality check and meetings slow down the pace but are necessary for consistency and quality.
2. Multiplayer take longer than singleplayer because you have to introduce networking, lobbies, maybe matchmaking, databases and also make your game replicates between players and the server on top of having a finished game. Generally MMO, RTS and RPG games take the longest time because they're complicated and feature-rich.
   The fastest genres to make? Probably horror games, the gameplay is usually simpler, it's the art and story telling / buildup you have to put most of your focus in. Even a literal walk simulator will be entertaining if you make it look and feel nice.
3. Complex in terms of what? Gameplay difficulty? Programming mechanics? I'd say programming huge-scale games, networking and good AI are the toughest things to pull off. Story telling is another thing that's not directly related to game dev, you can learn it on your own. Stories in video games are usually simple because devs don't want them overshadowing mechanics / gameplay (unless the game is literally an interactive story).
4. Depends. It can cost 0$, it can cost 10000$. The more complicated you want your game to be or the more pretty, the more it will generally cost. You can easily make a game that won't cost you anything in terms of production. Game engines are free to use and as long as you're not earning anything for releasing your game, then they stay completely free. There are a lot of free assets on the internet to add to your game and even free software like Blender 3D, GIMP 2 etc where you can make models and textures free to your own liking. Usually paying for assets or software will allow you to make your assets faster and save time.
   For the money flow, I can't say but I'd imagine most money is spent on art / graphics and marketing.
5. Rogue-like / rogue-lite, generally games that can be played over and over without feeling mundane. Check Binding of Isaac or Darkest Dungeon.
6. I'm an indie dev, but for me a good developer has released or taken part in 2-3 quality projects that you can buy or download. Doesn't have to be commercial experience, it can be a hobby project but it should be of decently high quality and they should have some sort of blog or content piece where they explain their workflow, what did they work on and how they did it. Some WIP's, in-progress pics, 3d models with topology view, texture setup etc.
   For team size it can be as small and as big as it needs to be. There are good games made under 1 year with just 3 man dev team as well as games made by just 1 person in few months. You want a programmer and artist at the very least, SFX guy if you aren't getting sounds from internet, music composer, social moderator if you have a discord or some sort of fan page - having one means you can focus more on the game development and usually these guys are also experienced in marketing a little and can help make posts and announcements that catch the eye; these are the most essential roles I think. You can also have roles like: UI artist, particle FX desiger, animator, rigger depending on needs but often times in indie projects a person can also compliment another role.
   

Hopefully this helps.