If Parliament is prorogued from 11th September to 14th October, how much time is there to debate and pass a new Withdrawal Agreement Bill, say with a modified backstop?
The leaked/open-mic comments of defence minister Ben Wallace clearly indicate that the Johnson government considers Parliament ineffective in Brexit matters, so intended to sideline it as much as possible.
Parliament has been very good at saying what it doesn't want. It has been awful at saying what it wants. That's the reality. So eventually any leader has to, you know, try.
For the record, the Johnson government has later disavowed Wallace on this, declaring that he "misspoke". Also, news as of Aug 29 are bi-weekly meetings between UK and EU Brexit negotiators, but that's about it:
After his chief negotiator, David Frost, met EU officials in Brussels on Wednesday, the prime minister said on Thursday that both sides had agreed to meet twice a week. [...]
According to a diplomatic note of Frost’s meeting in Brussels, seen by the Guardian, Frost downplayed Johnson’s suspension of parliament, describing it as normal. He told his interlocutors that Johnson wanted a deal but was not afraid of no deal.
The UK government appears to be seeking to convince the EU that it can bounce parliament into accepting any rewritten deal. Frost told EU officials that it would be possible to ratify a Brexit deal in the second half of October and argued that a technical extension would not be necessary. This strategy matches Johnson’s pledge to leave the EU on 31 October “do or die”.
EU diplomats remain sceptical that a solution to the thorny question of the Irish backstop can be devised in a mere 62 days.
If the Johnson government is somehow serious and does present a credible alternative to the backstop within 30 days, as demanded by Berlin and eventually echoed by Juncker, then Parliament might be given and opportunity to rubber stamp it. But a lot of observers consider this scenario implausible. The blame game is now really in full swing; neither side really wants to be seen as having completely closed down the negotiations and thus triggered the no-deal scenario.
The news as of 50 mins ago (as I'm writing this) is the EU (considers) it hasn't seen any "concrete proposals" from the UK.
It was mentioned on the BBC (live) a few minutes ago that if Boris Johnson forces a last minute deal at the EU Council (during the planned prorogation) then the parliament would have 6 working days to act/enact it (after the prorogation is over). I haven't double checked if this schedule makes sense. It was a commenter from "off" (not on screen) that made this remark, so I'm not sure who he was.
If some miraculous 11th-hour deal was to happen between the UK and EU to draft an alternative withdrawal agreement that both sides could/would agree to, then I am sure they would find a way to extend the Brexit deadline, so the new agreement could be passed by the UK Parliament. It seems that pretty much all it would take to have another extension is for Johnson's government to ask the EU and for them to say 'Yes'. They could probably have the required documents on standby in the negotiating chamber at 11pm on October 30th.