Hi Maunaloa, and welcome to Boshy speedrunning!
The sole reason why Dark Boshy is used from the start in all categories except 100%, is because the community decided the game was more fun to play that way.
If you want to use Dark Boshy without having to play through the game and collect the various chests required in order to unlock him, you can go to the "Resources" section to the left, and download the "Dark Boshy" file. Replace that file with the "onlineLicense.ini" file in your Boshy folder, and Dark Boshy will be unlocked the next time you boot up the game.
Feel free to ask any questions you might have. Also, Tehjman1993 has a good platforming tutorial in the "Guides" section, but these mostly apply to the Average mode route, and many saves have several ways to approach them.
-Famarok
My knowledge of NSC is very limited. But from looking at the leaderboards, we see that the current WR (17:06 by Pokeguy84) was done on a Gameboy player, so I would assume he used a+b+start+select. You will most likely be better off asking him directly however. Also, a fair share of the top runners of NSC are very active in the community and should be quite easy to get ahold of through Twitch or Twitter.
You generally need to hold down the keys for a few seconds before the soft reset takes place. In some cases it will take longer as the game can not be soft reset during certain circumstances, such as when a Pokémon is currently fainting.
This is a submission to race fellow Boshy runner BBF, who has his post on page 1. Therefore I will not go into any details regarding the description or bonus content. See his post for reference.
Game Name: I wanna be the Boshy
System: PC
Category: any% (Dark Boshy, no Wrong Warp)
Gameplay Estimate (excluding ending and bonus content): 40min
Ending/Bonus Estimate: BBF posted some incentives and he's way more experienced with those than I am, so he would be the one doing them.
Possible Donation Incentives: Same as above.
Sample run video (strongly encouraged but not required):
Description: See BBF's post.
Availability: Same as BBF (from Friday evening (~8 pm CET) to Sunday evening (~10 pm CET), but I would prefer to not run in the middle of the night for the sake of my sleeping schedule.)