Conditions : thirsty and hungry => only to be sure : you immediatly take the damage to attribute when you become thirsty and hungry? For example with a magic mishap.
There is a list on page 111 on the effect of Hungry and Thirsty. When it comes to Hungry, then as soon as you become Hungry (after a day without food normally), you will suffer one point of damage per week, so it will take a week before you take damage. When you are Thirsty (normally after a day without water), you will take damage to both Strength and Agility every day. This means that after a day with the Thirsty condition, you will start to take damage.
Recover from a condition : you only roll for food/water or automatically decrease your dice because you were thirsty/hungry?
As soon as you have eaten something you are no longer Hungry and as soon as you drink, you are no longer Thirsty. There are no special rules about this, just eat or drink something like you normally do.
Using spells in animal form : you can or cannot? Up to GM?
Yes you can. The spell is Personal, which means that the only one that you can cast this spell on is you and you need to cast the spell on yourself again to shift back.
Clothes and equipment in animal form : all the stuff on the ground or metamorphosed in you? Up to GM i guess?
The spell doesn't say. I would say it drops to the ground since I don't like this metamorphosis thing, but that's me.
Animal form : if strength is decreased, you get the same decrease when you change (to animal or normal form)? How does it work?
If you have taken damage in animal form, you retain the damage when you shift back to your normal form (should probably heal yourself first in some cases).