Does GAE-Search keep the field name with each document or at the top level? If it keeps (and transmits) field names with each document, I will need to minimize the field name length to reduce storage and network cost, else I can keep the names more readable.
We store field names with each document. So theoretically you could save a bit on cost by keeping the names short. But it seems like a tiny factor to be worrying about: generally the size of fields is dominated by content, compared to names. And of course this is an undocumented internal implementation detail, so it could change in the future.
By the way, I am indeed a "Google Search API engineer" (as you put it).