When you have a word that starts with a nasal followed by a plosive like in mche and mti, does the m usually become syllabic by itself, like in the audio here? Or is it usually pulled into the following syllable a bit more? It's probably the former, since that's how it sounds in the audio and if it were one syllable, the consonant cluster would go a bit against the sonority hierarchy.
I think just "seedling" alone should be accepted cos you can't have seedling of any other thing - there's nothing like: stone seedling, house seedling, car seedling, etc. The idea/concept of a seedling is only restricted to trees. Therefore whether one translates "mchea wa miti" as "tree seedling" or just "seedling" alone, both are correct and ought to be marked as such by Duo