A nice idea, and the net result of having the Lancaster available and trouble free earlier is a good one to contemplate, but does little to change the overall strategic outcome, despite less Bomber Command losses from 1940 onwards as a result of a greater availability of more reliable aircraft. Bomber Command still needs to address its accuracy issues and lack of numbers of heavy bombers that it had in 1940 in order to wage a successful strategic campaign, issues that standardising on the Lancaster earlier is simply not going to change in the short term.
Having the Halifax, Stirling and Manchester, despite their faults increases numbers, and if we were to somehow magic these types for Lancasters doesn't change the fact that the command was underprepared to wage the war it was expected to do so between the beginning of the war and around 1942 to '43. Until the advent of electronic navigation aids, the bombers are still fumbling around in the dark and their bombing accuracy doesn't improve, let alone finding their way to the target area. The Butt Report highlights these inefficiencies the command was suffering from up to then. Also, the placing of Harris as the command's head was perhaps the biggest influence on its improvement going forward and he energised the command in a way that his predecessors had not done before.
A few points:
1. The Leeds will be massively inferior to the Lancaster. Even if Hp make the change to merlins as in OTL we have a much inferior plane.
Obviously. The HP.57 which became the Halifax was based on the drawings and work done on the HP.56, Handley Page's entry to P.13/36 which I am assuming is what you are using as the model for the Leeds and of course, we know that the Halifax had a number of flaws that took years to iron out that affected its performance and increased its attrition rate. We can assume that the Leeds would have suffered similar issues simply by looking at illustrations of what the HP.56 was going to look like.
112-1.jpg (669×382) (aviadejavu.ru)
From here:
Air Pictorial 1955-04 (aviadejavu.ru)
2. The Lanc would enter service prob a couple of months before otl manchester Nov 1940 as there would be less teething problems.
I doubt this sincerely, simply because with all the issues the Manchester suffered, the Lancaster didn't as a result of them being solved before the four-Merlin engined Manchester Mk.III became a reality. Simply replacing the Vultures for Merlins wasn't going to remove the aerodynamic instability that rotating the FN.7 turret induced, the aircraft would have still suffered directional instability that resulted in the installation of a third central fin, which was eventually cured by increasing the size of the vertical stabilisers, but these were installed on the Lancaster prototype first
before installation on Manchesters on the production line, so an earlier four-engined Manchester would have suffered the same issues until worked out. The Manchester also suffered electrical and hydraulic issues that had been sorted by the time the Lancaster was put into service. The simple fact was that if it weren't for the failures of the Manchester, there wouldn't have been a successful, relatively failure free Lancaster.