Yes, Heinkel could have been a more successful manufacturer.
I think few would argue, that the He 177B (277 or 274) would have been a more effective aircraft for the LW., than the coupled twin of the '177'.
The He 100D in service would have made an interesting contrast to the '109', faster, and with a longer range. But with a smaller wing (span area), it would be less manoeuvrable than the 109!
Although the aircraft was improved, over the 100B, by increasing the span by 4 ft, and dispensing with the surface evaporation - supplanted by a semi-retractable belly radiator - its only service was with a squadron of Heinkel test pilots for the defence of the Rostok-Marienehe Heinkel factory.
As to the FW 187, I think many would say it deserved a better fate. In many respects it was similar to the Gloster G.39 (size, power performance). Only the first and second prototypes were single seaters, subsequently two-seaters as per RLM requirements.
But either way, how much better would it have been instead of the Me 110, in say the BoB - not a lot. Yet IMO I think the Ju 88C would have fared even worse than the Me 110, if it was used instead.
The Me 110 was originally intended for a specification that included light-bombing duties (much like the French Potez 630) - hence the need for a rear-gunner. And in that capacity it might have been more valuable in the BoB, rather than acting as long-range escort.