The Jedi honour system in duels and practice

Jedi aren't honorable. They uphold peace and justice, and they do what is necessary to be able to. When they come up against a threat to peace, they deal with it, using as much force (no pun intended) as they deem necessary.

TPM:
-Obi-Wan and Qui-Gon Vs. Maul - 2 on 1

Maul had shown himself to be a threat when Qui-gon fought him before; Qui-gon was severely winded after the battle. Add to that the fact that Obi-wan, though supremely skilled, was still just a Padawan, and it's quite understandable why they attempted to take him together.

AOTC:
-Mace Windu beheads Jango Fett.
-(intended) Obi-Wan and Anakin Vs. Dooku - 2 on 1

Jango was a known Jedi-killer. He had the skills and the know-how to take almost any Jedi head-on. We see two examples of this in the movie; once when Jango holds his own against Obi-wan (who is one of the best combat Jedi in the galaxy), and once when he completely destroys Coleman Trebor with a single blaster pistol. Mace had to take him down any way he could; with Jango on the ground, jetpack broken, and with a single blaster, it was the perfect opportunity to put the threat to rest. So Mace took it, and probably saved many of his Jedi brethren.

Obi-wan knew that Dooku was more than a match for either him or Anakin alone. Capturing or killing Dooku was their objective, and their best shot was to take him together. So, Obi-wan wanted to do so. Anakin had other ideas, and because of his headlong rush into "honorable" battle both he and Obi-wan would have been killed if not for Yoda.

ROTS:
-Obi-Wan and Anakin Vs. Dooku - 2 on 1
-Obi-Wan shoots Grievous
-Windu, Fisto, Tinn, Kolar Vs. Sidious - 4 on 1
-Yoda beheads guards (I think).
-Obi-Wan's 'high-ground' move.

Same thing here. Obi-wan knows that Dooku is still a force to be reckoned with, and he believes that their best chance lies in taking him together. So they do.

Grievous was the supreme commander of the Separatist army. He was brilliant, malevolent, and utterly devoted to destruction. He needed to be taken out, and Obi-wan's only chance was the blaster. So he used it.

I half-expected Mace to march after Sidious by himself (he seems arrogant enough), but he was intelligent enough to bring help. Darth Sidious was a being of such power that he trained two proficient Jedi-killers, fooled the entire Jedi population into thinking he wasn't even Force-sensitive, and masterminded a war designed to give himself ultimate power. He was the biggest threat the Jedi had known in at least a millenia, if not more than that. To send a single Jedi after this man would have been the height of idiocy.

I don't think Yoda beheaded any Royal Guards; maybe some of the clones standing guard at the Temple, though. However, the guards would have killed him just as quickly; in fact, they had orders to kill any Jedi on sight. The Jedi needed access to the Temple, and so Yoda got it.

Obi-wan's high ground move was quite honorable (although it was a RETARDED ending to the fight. Sorry, but I can't think about that stupid ending without getting angry); he even warned Anakin, tried to dissuade him. Obi-wan fought very honorably against Anakin, perhaps the most honorable battle in the saga.

 

I think whoever said Yoda beheaded the guards is just confusing the royal guards with the clone troopers on Kashyyyk. Yoda just force pushes the royal guards out of the way to get to Palpatine, while he does behead the clones trying to sneak up behind him to kill him. Self Defense, hardly dishonorable.

 

Yes, there appears to be confusion there. And you're right, Yoda was only defending himself when the Clones were about ready to shoot him in the back. If Yoda were an out-of-control tyrant, he would've killed the Royal Guards instead of just pushing them up against a wall.