Is it permissible for someone who prays Witr three continuous rak'ahs to sit for the tashahhud in the second rak'ah by mistake, then complete the third and make the tasleem, or to make the tasleem after the tashahhud and then perform the Witr rak'ah? And which of the two is more correct? Is sujood as-sahw required if one forgets the qunoot supplication? And is there qunoot when making up Witr after Fajr?

Connecting and separating in Witr prayer is valid, but separating is better to avoid resembling Witr to the Maghrib prayer. If the worshiper intends to pray Witr with three connected rak'ahs, then sits for the tashahhud (definition) mistakenly after the second rak'ah, he should stand up to perform the third rak'ah and then prostrate for forgetfulness (sujud (definition) as-sahw). Alternatively, it is permissible for him to change his intention to two rak'ahs, then perform the third. Changing the intention from an even number (shaf') to three, or vice versa, does not harm the prayer. Omitting the prostration for forgetfulness due to an extra sitting does not invalidate the prayer. The Qunut supplication in Witr is a Sunnah (definition) according to the Hanbalis throughout the year, and if one omits it by mistake, he should prostrate for forgetfulness. When making up a missed Witr prayer, it is prescribed to make it up in its original form, and the Qunut supplication should be recited if it was prescribed. Making up a three-rak'ah Witr prayer as four rak'ahs is permissible according to some scholars.