If a card that isn't a transforming double-faced card becomes a copy of a card with craft, it'll stay in exile if you activate the craft ability. It won't return to the battlefield.

If some of the creatures are illegal targets as the first ability of Jade Seedstones tries to resolve, the original distribution of counters still applies and the counters that would have been put on the illegal targets are lost. They won't be put instead on a legal target.

If the materials required include multiple objects, you may exile some of them from among permanents you control and the rest from among cards in your graveyard. You don't have to choose all permanents or all cards from your graveyard.

In some unusual cases, multiple cards may be exiled to craft Jadeheart Attendant. For example, if an effect makes Jade Seedstones a copy of a permanent with a craft ability that requires multiple components, the copy effect will expire as Jade Seedstones leaves the battlefield and it will return as Jadeheart Attendant. The enters-the-battlefield ability will count all the exiled cards, and you'll gain life equal to their total mana values.

The back faces of some cards with craft refer to cards "used to craft" it. This refers to the cards exiled as part of the cost of the craft ability of the front face. Those cards are considered to be "used to craft" that permanent as long as they remain exiled and the permanent remains on the battlefield, even if the permanent's controller changes or some of its characteristics change (because of a copy effect, for example.)

You choose how many targets the first ability of Jade Seedstones has and how the counters will be distributed as that ability is put on the stack. Each target must receive at least one counter.

You may exile tokens you control as part of the materials required. However, because they aren't cards and won't stay in exile, any abilities that refer to what you "used to craft" the back faces won't refer to anything.