Perfective and imperfective forms

I cannot agree with you about lemma and derivations. They are not forms, they are separate words. Nouns, adjectives, adverbs are also formed using suffixes and prefixes. This info useful in etymologies, e.g. сделать = с + делать but it doesn't need to be classified as a perfective form of делать, they form a pair. In many cases, the perfective verb is much more common and would be defined as lemma with your criteria (чокнуться) or may not have an imperfective form at all (кануть) or they use suppletive forms - сесть/садиться.