A few things seem so solidly stated in scripture as to be safe to base an answer on.
(1) The text in question has "if possible", and there's no getting away from that.
(2) Other scripture texts dealing with this exact, future time shed light. More later.
(3) To be deceived is not the same thing as losing one's salvation, as deception of 'the elect' (the chosen) may be with regard to matters separate from salvation.
First, then, the Greek for 'possible' in the text is dunatos, in the sense of 'able, capable' It is the same in the parallel text of Mark 13:22. Interestingly, there is the Greek word adunatos for not possible, but that is not in the text. Had Jesus intended to make an adamant statement, that it was not possible for the elect to be deceived at that future time, he would have said that, surely? This means that the 'if' is important. There cannot have been a scribal error, omitting the 'a' before dunatos, given that the word for 'if' is related to dunatos.
Of further relevance is how Jesus used the same phrase, "if possible", in his prayer in Gethsemane: "My Father, if it is possible, let this cup pass from me" (Matthew 26:39 & Mark 14:35-36). Of course, it was not possible because the will of the Father was to have his Son sacrificed at that time, which is why Jesus went on to add, "Yet not my will, but yours be done." This indicates that the author, Matthew, who wrote both phrases expressed by Jesus at different times, had the same intended meaning. This deals with the 'authorial intent' tag.
(2) Noting how Jesus spoke about "false Christs, and false prophets, and shall shew great signs and wonders" at this future time reminds me instantly of what the risen Lord said later about the time just before his sudden return to earth, when many would be deceived by great signs and wonders. This is in Revelation 12:9 and chapter 13. Twice we are told of a time to come when satanic, beastly powers on earth will "deceive the whole world" and "deceive them that dwell on the earth". How? With lying signs and portents. It is Satan who is the invisible power behind the devices and systems he has, by then, put into operation all over the world, in order to make people give false worship.
Yet the prophecy also assures us that God's elect have, by then, been identified; only "the world" of mankind that has the mark of the beast continue to be be so deceived, and only they will be adversely judged at Christ's awesome appearing. That beast who blasphemes against God is allowed "to make war with the saints and to overcome them" (13:7) yet (3) their being overcome does not mean they are deceived! Many in the book of Revelation are martyred for their constant faith in Christ. They remain faithful to the last despite being overcome on earth by satanic blasphemers, false prophets, and deceivers. Back to (2). Notice what this second beast (out of the sea) does:
"And he doeth great signs, so that he maketh fire come down from
heaven on the earth in the sight of men, and deceiveth them that dwell
on the earth by those signs which he had power to do in the sight of
the beast; saying to them that dwell on the earth that they should
make an image to the beast which had the wound by a sword, and did
live... and cause as many as would not worship the image of the beast should be killed." Revelation 13:13-17
Note that God's elect do not give this false worship and they are killed for that reason. But all those with the mark of the beast continue to be deceived.
Jesus told his followers in Matthew 24:24 of a terrible time of trouble, of great tribulation, that would come before the harvest of the world begins (29-31). But God is sovereign, and will not allow satanic deceptions to continue for so long as to see the deaths of all the elect. Satan will not be allowed to run the great tribulation deceptions for such a length of time as that. It was the risen Christ who gave the apostle John the final visions of that time. Put what he said in Matthew 24:24 with what he revealed about just before he returns, and there is perfect consistency.
(3) The elect will suffer great tribulation, but that does not mean they must be deceived. The elect will not give false worship and many will die for that reason. No matter what they make of false prophets and lying signs and portents, at the last they will refuse to worship the image of the beast (the intent of the deceivers). And those who know the prophetic scriptures well will be in the best position to identify those deceptions from the start. They will also know that God will not allow that time of tribulation to go on for so long that they will all be killed for their faithfulness.
In summary, and to answer your final question, "is "if it were possible to deceive the elect" a figure of speech intended to be understood as "it is not possible" or are the days of tribulation shortened in order to preserve them only to allow (some of) them to then be deceived?" - the answer to both those questions is, "No." There is a third possibility which I have striven to show; that the elect - at the end - will not be deceived into giving Satan worship, no matter how confusing and terrifying the times. Giving false worship ("strange fire" Numbers 3:4 cf. Rev. 13:13) at that time will result in damnation. None of the elect will do that because of knowing from the scriptures what's really going on, and because of knowing their Lord and Saviour. The elect will be prepared to be martyred rather than fall for the deceptions that the whole world falls for.