Wand´s ownership

If the wand´s ownership works as told by Olivander, each time someone was defeated or disarmed, the ownership of his/her wand would be transfered to the winner. In that case each time they used Expelliarmus against Death Eaters or against themselves when practicing the owners of the wands would have changed, making every or almost every wand work worse than before because the holder wasn´t the true master of it. Or where the cases od the Elder Wand with Dumbledore and the wands stolen from Malfoy exeptions, because if not, the wand issue would be a problen to the whole magical comunity...

i think that spells like expelliarmus that are just intended to temporarily disarm the opponent dont change the wands allegiances (like ollivander says: the wand chooses the wizrd).

The new ownership thing only applies to the Elder Wand.

no, it was harder for hermione to use bellatrix's wand b/c she had not won it, and the same for harry when he tried to use the wand ron had given him. olivander said that a wand needed to be won, but not nescisarily kill the previous owner.

When they practised the expelliarmus spell, they didn't intend to take the wand for ever. If the previous owner takes it back immediately afterwards, you haven't won it.
In Dumbledores case, he died before being able to take it back, so it would belong to the one who disarmed him.

What I still can't understand though is why Harry would become master of the Elder Wand, by taking Draco's other wand. But I guess there are some sort of connection between all wands having the same owner...

i think its more of defeating the owner than the wand to be honest! cause harry defeated draco gaining his wand he was the owner of dracos wand but he didnt realise that he was the owner of both wands