Divine bond, used for Defending. What is the duration?

[Mulet]

If I use Divine bond at level 8 (+2), I can turn my mundane sword into a +2 sword for 80 rounds.

At any round after that, I can then use defending on the start of my turn, dropping my sword to a +1, but getting +1 AC.

On the turn following that, does my Sword return to +2? Or is that bonus consumed? And my +1 AC on that turn is also gone, yes?

[Bob Bob Bob]

So there's two things here. First, you need your weapon to already be defending or you need to add defending to the weapon (so you would add +1 defending instead of +2). Second, your question is answered in the description of defending.

Defending wrote:

A defending weapon allows the wielder to transfer some or all of the weapon's enhancement bonus to his AC as a bonus that stacks with all others. As a free action, the wielder chooses how to allocate the weapon's enhancement bonus at the start of his turn before using the weapon, and the bonus to AC lasts until his next turn. This ability can only be placed on melee weapons.

So you choose before making an attack (as a free action) how much of your weapon's enhancement bonus to move to defense for that round. You can move all of it or none of it. Next round it's all back to normal and you choose again.

[Sonea Evareska]

Divine bond wont work like that. When you activate it, you decide which abilities you want to go into effect.

At level 8 you would have a maximum of +2 bonus to use. With a non-magical weapon here is what you can do.

First you make it a +1 weapon. Afterwards you choose:
A) another +1 enhancement for a +2 total or
B) you have a choice of adding one of these keen/defending/flaming/merciful

You get the bonuses for 8 mins and then your weapon reverts to its pre-bond stats. No switching of bonuses during the bonds duration

Edit:
And totally missed the last part, but yes your weapon is +1 defending again before you decide to switch the bonus.