Can I replace the home button on the  iPhone 3G without removing the glass digitizer?
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.