Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

In 5e, instant death on a five wound model counted as five toward combat res (even if only one wound was inflicted)

4 wounds inflicted on a one wound model counted for one.

How are things like this handled in 6e now?

Exactly the same. Unsaved wounds in excess of a models wounds are lost.

And, if a model with 5 wounds has one ID wound allocated to it, how many are added to results?
(unsaved wounds inflicted = 1, right? )

And, if a FAQ says wounds from challenges don't get allocated outside challenges, a character can contribute only as many wounds as the other character in the challenge has ?

ID wounds = the number of wounds lost. Page 26, second to last paragraph under Determine Assault Results.

In other words, combat resolution counts all wounds actually lost. The exception is in challenges, where it says:

Assault Results, Page 65
"Unsaved wounds caused in a challenge count towards the assault result, alongside any unsaved wounds caused by the rest of the character's unit."

Note, it does not specify wounds actually lost, only unsaved wounds. You take wounds all at the same time in a challenge (since there is rarely mixed saves involved). All that are unsaved count towards combat resolution as it states above.