My user dict is always a copy of the flashcard db, but I will copy the flashcards to main storage because this is the valuable data. I make a copy of flashcards.pqb about once a week ...
There was no great difference in speed importing the flashcard export to to user dict either with or without unsafe mode, but your are right: the import happens only once.
Any dictionary sort order in Manage Dicts seems to lead to the same result: e.g with
ACED
Unihan
ABC
Oxford
etc.
I only get ACE in compounds. All dictionaries below Unihan and including Unihan are invisible in compounds. I marked all 4 ways of use for all dictionaries (show in menu, use in toolbar, ...). The more I move down Unihan, the more dictionaries are visible in compounds. I have ACED, ABC, Oxford, Tuttle, Guifan, Handedict, cc-cedict, adso, an upgraded User dict for classical chinese (from a forum member), radicals and 2 user dicts, both created within the last 2 weeks, one of them only for testing Beta 2. On the SD card in the Pleco folder are all dictionaries, the fonts Pleco Extra and ZYSong, the classical chinese user dictionary and a file Radical_Dict, last modified today, which I do not remember where it came from (there is a p2radicals file too). The other 2 user dicts are in MyDocuments.
When I exported my 6 test flashcards (to test Card Filter not correct in row) as XML file and opened this file in Excel, I expected 6 records or lines in Excel, but actually I saw about 14, some flashcards had 1 line/record, some flashcards had 2 or 3 lines/records. But if you include card history in Beta 3, I will probably no longer search for XML details.