No point in sending anyone then.
Anyone with any cop on to the scenario at the time knew that was all they were going to get.
Dev knew it for one and he wasn't alone
Collins knew it as well but also knew his war in Dublin was under serious pressure.
And that's the point. If anything was to be had from the negotiations it would have taken a keen negotiating mind. Lloyd George had described dealing with Dev as like trying to pick up quicksilver with a fork. Dev knew there was likely to be some hard bargaining by the british and a 32 county republic was very unlikely to be acceded to - so rather than go himself and come back with a partial prize, he sent Collins over as a fall guy knowing it would be highly unlikely that anyone would come back with what Dev wanted.
Collins knew he wasn't the best man for that job. But he was sent, by a cabinet controlled by Dev. And so he did his duty.
As for his war being "under serious pressure" his war was to make Ireland ungovernable by the british, not to expel the british. Thanks to public opinion and british stupidity his campaign in dublin was doing what it set out to do.
Though Dev's ego trying to insist on large scale engagements would have ground it all to a halt had he gotten his way. Stupid enterprises like burning the Customs House in late May 1921, just weeks before the Truce, where over 100 of the Dublin brigade got lifted.