Yes, the author is stating "we" "us" etc.
IMO this because one Jew is adressing an audience of brother Israelites.
But chapt 10 alone makes it perfectly clear....they were at risk of falling back into apostasy because of willfully rejecting Jesus and the New Covenant and maintaining the obsolete and ineffective Old Covenant. Really the scope and thrust is overwhelming and unmistakable. Apostasy was the imminent danger that is being warned of.
Yes it is about apostasy
Verse 22 says, "let us draw
near with a true heart in full assurance of faith..." Verse 23 further encourages us to "hold fast" of our profession of faith without wavering.
Then at the end of the chapter, we are told not to cast away our confidence of faith and "
draw back" for God has no pleasure to those who do. The "draw back" is a reference to those people who waver and leave faith and go back to their own works, ie leaving the covenant of grace and going back to the covenant of the law.
So, verses 28-30 are just dealing with someone who does that. The idea here is that if you leave the covering of the blood of Christ (leave the covenant of the promise)
then there is nowhere to go for your salvation. ...there are no more sacrifices for sin... the system is done.
Just look at the references to the law in those verses.
Verses 28-30 deal with the idea that if God dealt severely with those who violated the Law of Moses, then how do you think He would feel about those who purposely leave the blood of the new covenant?
However, this is not about loosing salvation but being faithful to the faith.
These Hebrews wanted to know if the blood sacrifice were still needed.