What level are the bosses in?

Iggy	Level 1F6
Morton	Level E5
Lemmy	Level 1F2
Ludwig	Level D9
Roy	Level CC
Wendy	Level D3
Larry	Level 1EB
Bowser	Level 1C7
Reznor (Vanilla Fortress)	Level DF
Reznor (Forest Fortress)	Level D5
Reznor (Chocolate Fortress)	Level E2
Reznor (Valley Fortress)	Level 1DE

How can I make a Lakitu/Magikoopa/Reappearing Boos stop following/spawning in after a certain point in my levels?

Place Sprite D2 in the level where you want them to stop. You can find this in Special Commands and Generators list of the Add Sprites Window.

How can I prevent fade outs at the end of levels?

For no fade out at the goal, use this hex edit/patch and insert it with Asar.

This hex edit/patch does not disable fade outs after boss battles.

Is there an easy way to fix a lot of SMW bugs?

Look through this link for bug fixes.

My changes aren't showing up when I play the ROM!

Don't load from a savestate, and make sure you don't have any BPS files with the same name as your ROM in your ROM's folder.

Can I rearrange the status bar?

The Status Effect tool can be used to change up the status bar. There are several patches that can be used to modify the status bar, as well.

Why are my overworld paths showing glitchy tiles?

You very likely never uninstalled the title screen recording ASM. Open the Overworld Editor and go to File > Title Screen > Uninstall Title Moves Recording ASM.