title_body: How to naturally evolve verbs into adverbs or dependent clauses?
upvoted_answer: This is more a lengthy comment on your question than a real answer, but comment space is restricted ...
  
  You are undertaking a very difficult task by setting a starting point and an end point and asking for natural development between the two. This is difficult even when it is known that such a natural development exists because the two languages are attested natural languages and not constructed.
  
  We know sound laws really well, and also the process of grammaticalization is fairly well understood. The evolution of different basic word orders is still under research, we know that it has happened but how exactly it happened, is less clear.
  
  It seems that your are needing some grammaticalisation to get from the verb pfufi to the adverb pfufe. Here's a suggestion: Add a particle ye meaning roughly "like" to your proto-language, and the phrase pfufi ye will fill the role of an adverb. It is easy to conceive, that pfufi ye evolves to pfufe by just dropping one syllable (syncope). To make tushu from tushi a particle like vu might be applicable, but I have some difficulties to define its original meaning.
  
  I think that language change is easier to "grow" than to "plot", i.e., it is easier to just apply changes to a proto-language and watch where they lead to than to interpolate between two stages of a language. 
  
  Of course you can invoke language contact to achieve a particular change in your language evolution, as kind of last resort to save the plot. Just don't overuse this.