2.) Helmets can disguise voices. Take your average villain calling you over the phone and disguising their voice with one of those gadgets. Think of the technology during the time of Star Wars; of course, they can make all of the clones sound exactly like Jango Fett. The Delta Squad also did not have matching voices, so I sort of believe in this theory.
3.) Jango Fett was able to kill a JEDI MASTER with ease. When 'he' flew down from the balcony, 'he' was disposed of easily by Mace Windu. With the look on Mace Windu's face, as well as Count Dooku's face, I think Mace realized it wasn't Jango Fett who he killed. I think this was a decoy so Jango Fett could escape.
4.) The only Clone Trooper I remember seeing that took his helmet off is Clone Commander Cody, who has a 100% likeness to Jango Fett.
5.) Commander Cody and Obi-Wan Kenobi seemed to have a long history. Who better than Jango Fett would know Obi-Wan Kenobi? Jango couldn't kill Obi-Wan Kenobi on Kamino. Jango Fett couldn't kill Obi-Wan Kenobi in Geonosis' ring (asteroids). Don't you think Jango Fett would be out for revenge? I am sure Darth Sidious realized that Kenobi wouldn't be easy-to-kill and Jango Fett would stop at nothing to get revenge on Kenobi. Of course, Kenobi never saw it coming because Jango Fett disguised himself as a 'clone.' Jango pretty much failed in killing Kenobi, as you can all tell.