My iPhone 3G home button broke so I bought a new one online and all the repair guides require me to remove the glass digitizer, which seems very tough (hair drying glue etc.) and harder than I expected. Does anyone know a way or a guide of replacing it without removing the glass?
No - if you can't effect a repair using a clear film or some sort of adhesive with the phone intact, you'll have to totally disassemble the 3G/3GS display unit to get better access to the home button.
The teardown guides from iFixit are accurate - as is your reading of the difficulty of successfully doing the repair and having things work when put back together.
Do check with Apple on the replacement price of that part before you dive in. Last I checked, it was $99 for parts and labor for an entire new display assembly.