Well, this got me pouring over the rulebooks again..
And I have determined it is definitely a... depends.
For the first question(bypassing DR)
There is nothing that says it 'loses' its status as a +1 weapon. It (could be argued) that it is still a +1 weapon, though it is not longer giving the +1 to hit/damage. The DR rule says that it has to have an enhancement bonus,and the defender does. No where does it say that it has to use the bonus for attacking. NOw, I can see how you may judge it to be that way; but I can also see how it makes sense to look at the weapon as 'powerful magic' and let it go as powerful enough.
In fact, upon re-reading the 'defender' description. It seems to agree. The enhancement bonus is tranfered from being 'to hit' to being 'to defend'; it talks about transfering, not removing. It still has a +1 enhancement; you just get to determine what it is enhancing.
As for the sundering of the weapon. It still depends, on the same beliefs. Personally, I still think it is a +1 weapon, it just give +1 AC instead of +1 to hit. (almost said THAC0
Of course, this also brings up if you consider 'other' properties to add to the sunder difficulty as they add to the price. Thus making your +1 defender the equivalent of a +2 weapon for the purposes of sundering. This makes plenty of sense to me, since it is 'more magical'/'more powerful' than a typical +1 sword.
If you say no to the last two, consider this. You have a +5 Holy Defender with Flaming burst. Pretty impressive.... But when you transfer the +5 to defense, I get to sunder your sword with my club. Huh??
Furthermore, I think the above decisions can be made almost independently. I would rule that your sword is *still* a +1 no matter how you transfer the enhancement bonus, but if you make it AC, it doesn't get past the DR. I don't know what I would do about the defender making it '+2'. But I think the rules can be used to argue either side on all three cases. (well, the last one is a bit weak.)
Sorry for rambling....
.
And I have determined it is definitely a... depends.
For the first question(bypassing DR)
There is nothing that says it 'loses' its status as a +1 weapon. It (could be argued) that it is still a +1 weapon, though it is not longer giving the +1 to hit/damage. The DR rule says that it has to have an enhancement bonus,and the defender does. No where does it say that it has to use the bonus for attacking. NOw, I can see how you may judge it to be that way; but I can also see how it makes sense to look at the weapon as 'powerful magic' and let it go as powerful enough.
In fact, upon re-reading the 'defender' description. It seems to agree. The enhancement bonus is tranfered from being 'to hit' to being 'to defend'; it talks about transfering, not removing. It still has a +1 enhancement; you just get to determine what it is enhancing.
As for the sundering of the weapon. It still depends, on the same beliefs. Personally, I still think it is a +1 weapon, it just give +1 AC instead of +1 to hit. (almost said THAC0

Of course, this also brings up if you consider 'other' properties to add to the sunder difficulty as they add to the price. Thus making your +1 defender the equivalent of a +2 weapon for the purposes of sundering. This makes plenty of sense to me, since it is 'more magical'/'more powerful' than a typical +1 sword.
If you say no to the last two, consider this. You have a +5 Holy Defender with Flaming burst. Pretty impressive.... But when you transfer the +5 to defense, I get to sunder your sword with my club. Huh??
Furthermore, I think the above decisions can be made almost independently. I would rule that your sword is *still* a +1 no matter how you transfer the enhancement bonus, but if you make it AC, it doesn't get past the DR. I don't know what I would do about the defender making it '+2'. But I think the rules can be used to argue either side on all three cases. (well, the last one is a bit weak.)
Sorry for rambling....
.