according to the recently announced, IEBC recruitment which just ended, DEPUTY R.Os temporary employees for at least 60 days earning 3,000 per day
don't know about returning officers

I thought the ball is more in the court of Presiding Officers manning polling station//streams than Returning Officer!!!!!

I am with you here...not with those who think in terms of IEBC being defeated by Nasa at the expense of Jubilee.
I love the area where the Lady Justice mentioned Kreggler report recommendations that saw all sides of the divide having more tallied votes than the registered voters. We have seen it happen in just concluded Primaries in Machakos county.
It is a matter of accountability and responsibility as PO has to verify what is on register before formal announcement at her/his level which he can object from powers bestowed.
This is not a win for Nasa nor is it a loss for IEBC, it is making things easy and fair.
It makes it easy for a voter to point a finger not at Chebukhati but to level PO
Remember Kivuitu answering questions with "I do not know as I was not on the ground"
Now we have Ground people to answer that
IEBC Senior officials just need to give their SOPs to PO and determine who will be where not Political parties.
Observers for political parties and others have a head to get information and his area must be secured from hooligans of political parties.
Those who are ready to say the elections are rigged will not have their say!

Is it the announcement of constituency results that is final or the signed document?

Lets say the results are completed at 2am in the morning and the Returning Officer in Kasipul Kabondo constituency due to a slip of the tongue announces to all gathered that Baba has garnered 120,239 votes. The tally that he has in his hand and which he signs shows that Baba actually got 102,239 votes.

So which is the official result, the announced result or the signed document?

Does IEBC do the final tally according to documentation or announcements?

That is a very valid concern @Limanika. After the counting votes most of the work involves tallying and verification of documents. This is very boring work and in my opinion requires a quiet atmosphere like an office with office equipment and personnel. In what facilities will Retuning Officers be housed to do their work but still allowing raia to be "Macho, ili kura yenu isiibiwe?"