I don't see any indication at all that Jesus is not being literal in verses 4-14.
It's literally things that come upon the spiritual Kingdom of God as the Gospel is preached unto all the world. Don't forget Christ is speaking to His disciples about things in this age that will affect them, and in fact many to be martyred for their faith.
Many professing Christians among the body of Christ will be deceived by those who profess to be of Christ, who are not.
Literal war and rumors of war have always been. But the warfare that affects the disciples during this age that shall bring the disciples "great tribulation" are the on-going spiritual battles being waged between the forces for good and the forces for evil.
Throughout redemptive history we read of battling of kingdoms against kingdoms, both literal/physical as well as the spiritual battles between Satan and his minions and Christ and His disciples. There have also been literal famines, pestilences, and earthquakes at variou times and in various places. These all come as a result of the greatest famine/plague is of hearing the Word of God. Refusal to hear His Word results in consequences in the natural world.
I don't think it says that in order to indicate what follows is meant to be taken in a spiritual sense rather than literal. If verses 15-22 are meant to be taken in a spiritual sense then what was Jesus talking about when He said those in Judea should flee to the mountains? What did He mean when He indicated that the distress/tribulation He described and the need to flee from it would be particularly difficult for nursing mothers and pregnant women and that it would be particularly difficult during the winter or on the Sabbath?
I said these things written must be spiritually discerned. I did not say they are not literal or physical.
With that said, I think He was also partly referring to a future time before His coming at the end of the age during which wickedness would be greatly increased as well as the number of people turning away from the faith. Which is something Paul wrote about in 2 Thessalonians 2 as well. I see passages like those as referring to Satan's little season.
I would say what is written if for the disciples to understand in all the ages from the first to the last. I believe there are two possibilities for how we should understand "
This generation shall not pass, till all these things be fulfilled." It can either be that the nation of Israel will always remain until Christ returns, or "this generation" as evil shall be with us until Christ returns.
Where did Jesus indicate that the sign they were asking about would be the Holy Spirit they would receive?
It's not found in the discourse but is found to be truth from other passages and verses. I believe the focus was not a sign for when Christ would come again. Rather it is the SIGN itself that would be evidence that Christ would come again and that this world would have an end.