The ball is in the EU’s court right at the moment.
They may choose to accept the unsigned letter and grant a delay. There’s nothing stopping them doing that.
They may choose to instead accept that Boris Johnson’s letter supersedes it.
They may just decide they do not care anymore and not pay any attention to any letter at all and just say “No more delays.”
It only takes one member state to veto the request for the extension and that is it. One member state to be fed up and say “Enough is enough.”
An extension can only be granted by the unanimous agreement of all the member states (except the UK - it can’t veto its own request, or I suspect Johnson would try doing that!) - if one disagrees, that’s the ball game.
At the moment - I wouldn’t dare make a prediction.
My hope is the UK gets out on the 31st October but I don’t feel confident one way or the other.