Two members have already given good answers to your question, but I will try to give another simple reply.
You have rightly interpreted its meaning that (in your own words) "the fact that it had been entrusted to another hand than that of the author is justified", but there can only be 'justification for' a noun, and what is the noun form of 'entrust?'
Sometimes we can unknowingly choose a verb for which there is no suitable noun form (or one that is not commonly known.) 'Entrust' seems to be one such verb.
If we should indeed use a form of 'entrust', then there is no one word to describe 'the state of having been entrusted' as a noun (while retaining a form of 'entrust' in the sentence) -- that is why the author has gone roundabout and 'justified' (the 'state or situation' of) its having been entrusted.
A whole lot of English has been written like this over the years, and may even be considered elegant by some people, including myself, but it is indeed roundabout and traps 'many an unwary reader' in its coils -- it is the main reason I have spectacularly failed at reading the Victorian classics, and many modern teachers of style (including the great Hemingway) would consider it old-fashioned, 'ponderous', cumbersome or just plain 'stuffy!'
Those who like plain and simple English could rewrite the sentence as follows (which you will then clearly understand without the roundabout 'fancy' constructions):
The present book is designed differently from any previous collection of Pound's essays;
so I believe there is justification for the fact that it was entrusted to a hand other than that of the author.''
EDITED after 1 hour:
Like others, I also thought there is no such noun as 'entrustment' but when I googled it, I found that it does exist, mainly in legal language; it is found defined in certain online dictionaries as meaning 'to entrust' -- please google 'entrustment' for details.