If Johnson can get enough Labour MPs to support an NI only backstop just to get the damn thing over with, he doesn't need DUP or ERG
Sounds easy enough, but it seems to have been Labour Policy since the 2017 GE to push for another GE. So they voted against the WA every time in the hope it would make the Conservative Party look bad and hopefully force a GE.
Labour got excited when they won 40% of the vote in 2017, but forgot that it was attributable to students being told that loans would be scrapped. Corbyn later said that it was not a commitment.
In every episode of Question Time since 2017 GE the labour MPs were arguing for a general election up until Farage's Brexit party topped the EU elections and labour's supported dropped from 40% to less than 20% at one point.
Although not completely committed to, or confirmed by, the labour party are now a remain supporter. So any WA put forward to parliament to leave the EU will not be supported by labour as they are a remain party. Their hope is to delay Brexit as long as possible so that it is forgotten and never happens.
So labour have effectively replaced the DUP and ERG in terms of opposition to WA, but for different reasons.
DUP wants GFA to be burned. ERG wants no deal. Labour want to remain.
All three result in WA being rejected.