A weaker leader could have been helped by a cabinet that united behind him. That, in fact, was their line whenever they pushed one of the clones. 'We know they're not really leadership material but we should unite behind them'. And then Corbyn won and they threw the toys out of the pram.
You are aware the majority of the cabinet was behind him?
After Corbyns election the party was in three groups.
1) Corbyn defenders. The small, far left wing of the party that had always revelled and evens thorn in the leaderships side,who were now in charge
2) the 'make it work' group - the majority of MPs. Not happy with the result but willing to try. Includes Andy Burnham. These have not been sabotaging him. They are the majority of MPs both left and right (remember Corbyn is not the sole leftie in parliament - there are left wing folks who disagree with him)
3) the 'kill it at all costs' group, aka The Blairites. Consists of around 20 MPs tops. Worth noting that several Blairites fall in camp 2, as they recognised it was a lost cause. They have very little power or influence right now.
The current issues are being caused by the entire 'make it work' camp, from both wings of the party, saying enough is enough. Not only are the polls dreadful for the party, but in the Remain campaign they got constant feedback that Corbyn was a dreadful leader on the doorstep, and that counts for a *lot* with labour MPs. Additionally, one thing that does bind all wings of the party together is a commitment to Europe, and Corbyns sabotage of that was the straw that broke the camels back for many MPs - they see it, rightfully, as an utter betrayal after they fell into line and worked with Corbyn after the result.
Anyone who thinks this is because the MPs refused to back him as leader and have been causing trouble ever since simply hasn't been following politics and doesn't understand how the labour part works. The party has an endemic fear of decapitations - this is unprecedented in modern times. This is solely on Corbyn - the guy is not fit to run a major party.