If you are in a clan, and other members of your clan donate troops to you, they will be stored in your clan castle and come out for defense. This is the only way that I know of to defend with troops. So no, you can't train troops and use them for defense. However, others may donate troops that you can use for defense. Also, skeleton traps may be considered troops by some, even though they are not trained.

No, you're not limited to the closest multiple of 5.

So, yes - You can choose to take only 10 people to war, even if you have more than 15 members.

With the update, as many people as possible are allowed to sit out, as long as the total number participating is a multiple of 5 and is more than the minimum requirement of 10 participants.

All you have to do is set who to exclude/include.

I'm in a clan of about 48/50 people and only 30 (18 sitting out) of us go to war at the same time.
The rest are either marked out, unlucky to be left out by trophy ranking cutoffs or marked as away whilst leeching.. Or are marked as in but excluded because they only attack with Barbarians and Archers...