Just keep in mind that the He 219 nightfighter is just another hate-pet of the MiloMorai like the Bf 109. All forums are loud from his whining and endless rhetorics what a poor plane the 'Uhu' was. Even if it`s actual combat results proved to be outstanding (iirc, the first combat sortie brought 5 Lanc kills in a single Uhu). As for barely catching the Mossie bomber version, those could do no more than ca 380mph, and there was no such moron pilot who would run at max speed, nor would this pilot reach it`s target before running out of fuel... cruising speeds were far less than that, not to mention that while enourmous amount is whining is put into pointing out the Uhu`s speed was lower when night-equipment was fitted like flame dampers (WOW, huge discovery), the same equipment had to be fitted on the Mosquitos (and Lancasters etc.) as well, hurting performance just as bad.
But besides that, I think it was correct not to put the Uhu into production, regardless the nice qualties it had. There was no need for another, specialized type airframe in production when the multi purpose and proven Bf 110 and Ju 88 could do the job very well against the heavy bombers, and not wasting resources to counter a high profile nuisance the Mosquito was. Moreover, GM-1 boosted Ju 88 G subtypes have the same performance the Uhu had, so why bother with it?
Now back on the orig topic, it`s nice if an airplane completes it`s tests, but it`s different from being ready to go into production. First thing, the engine manufacturers may not keep up with the demands, this is quite usual thing to happen. Just think of the Me 262. Engines were not ready, neither yet mass produced.. And as for the 410s etc getting the 603 engines, keep in mind that in early 1943 there was hardly any opposing fighter design that would prompt for the immidiate production of such uber-performance fighter as the FW 190C.. at best the enemy had equal technology. Fast bombers like the 410 were however very usuful in their everyday tasks, if not so glorious as the shining fighters, but it`s always the bomber that wins the battles!