Door to door is for the most part discouraged when there is any other reasonable form of finding people to teach available. The stats I have seen is that on average it takes 100,000 doors answered for a conversion to occur. Contrast this to teaching someone that has a friend that is in the Church and (the important part) the friend is present. This gives a success rate of something like 1 in 12 or so.

Half of all missionaries from any given country serve within the country of origin. This doesn't by itself contradict anything you said but is interesting.

zealots are more valuable than the lukewarm

Depending on what you mean then you are either wrong or right here. Actually practicing the religion and believing and seeking out to know, if considered to be zealots, then you are right. Unthinking practicing is discouraged which is generally part of what people mean when they use zealot.