There are many topics mentioned in the 27 Psalms you reference, and just because every single one of the ten plagues are not mentioned does not mean that there is a discrepancy. The Psalms are poetic and are an artists method of praising God, retelling parts of history, and prophesying. Thus they are not always complete in the information presented due to the nature of putting words to music. As for Amos 4:6-11, those were plagues dealt to Israel and not Egypt. As for the book of Revelation, there are actually 14 plagues when you consider both the trumpet and bowl plagues. There are also additional plagues found in the seven seals. Though some of them will be similar to what Egypt experienced, nothing in the Bible says that there must be an exact match to everything that God does or will do. There are many types that portray future and past events, but it is not always precise in every detail. In addition, the books that you mentioned such as "Apocalypse of Abraham, the Book of Jubilees, Philo's retelling of the Egyptian plagues, Artapanus, Wisdom of Solomon, Testament of Benjamin, and Greek Apocalypse of Baruch" are part of the pseudepigrapha which are spurious works that cannot be trusted. I hope this helps.

Regardless of its original source, the biblical text we have today was a fluid oral tradition for a very long time before it was written down and codified. It demonstrates influences from neighboring traditions and was subject to the agendas and aesthetics of the priests and poets who were the story-tellers and scribes of old, all of which helps account for the many discrepancies in the text. We can tell by the styles of language (as one can tell the periods of Beowulf, Shakespeare and modernity apart) that what we now have was not written down by one hand, or all at once. Either some discrepancies weren't caught , or the scribes intended to be thorough by being inclusive--perhaps some of both. What counts is the tremendous effort that went into preserving and codifying the oral tradition, and the impact the written text still has in modernity.

In Hebrew, one will have to know the function of the ordinal and the cardinal numbers. The ordinal number from 3-10 in Hebrew are nouns, 1 is an adjective and 2 is a noun, but unlike 3-10 it agrees in gender, while 3-10 does not. You see all of the numbers have feminine and masculine gender. I do not have my Hebrew Bible on hand but from a deduction, I would say it seems as if the numbers are not cardinals in Psalm 78 and 105 but ordinals. Therefore the 5th, 6th, and 9th plagues were omitted. Cardinal numbers are used for counting (one daughter, two daughters, two of them, two of us, etc.), while the ordinals are used to indicate a position in a series (first, second, third, etc.). The only plagues that was not mentioned in Psalm 78 and 105 were 5. murrain of beasts, 6. boils and blains and 9. darkness. The reason I deduct from this is that 5, 6, and 9 are the only ones that agree as cardinals and ordinals because they have similar masculine roots. The others do not. So this would indicate that 5, 6, and 9 can be used for counting or position, while 1, 2, 3, 4, 7, 8, 10 were mentioned because they indicated the ordinal only and not the cardinal. God Bless!!!