The trie determines what is “in vocabulary” and the valid_word_count_weight determines how much importance should be given to the trie’s opinion on what is in and what is not in vocabulary.
So, in particular, increasing the value of valid_word_count_weight should decrease the occurrence of out of vocabulary words as defined by the trie.
The weights I mentioned lm_weight, word_count_weight, and valid_word_count_weight are external to the language model and are not part of the language model weights which, as you mention, are built automatically.
I hope that’s a bit clearer?