The OP asks, “Is ‘if’ a hypothetical or real possibility in Hebrews 10:26?” Based on the grammatical construction of Heb 10:26, the verse is describing a real rather than a hypothetical possibility. The first thing to note is that the “if” in the ESV and other English translations is not in the Greek but is inferred from the context.
Hebrews 10:26a Interlinear (Note: not taken from interlinear of biblehub.com)
εκουσιως γαρ αμαρτανοντων ημων μετα το λαβειν την επιγνωσιν της αληθειας
willingly for sinning we after - to receive the knowledge of the truth
Hebrews 10:26 YLT
For we -- wilfully sinning after the receiving the full knowledge of
the truth -- no more for sins doth there remain a sacrifice
Syntax and construction
The first half of Heb 10:26 is constructed as a genitive absolute, a participle phrase wherein the participle, which is in the genitive case, functions as a verb with its own subject, one that is distinct from that of the main clause. Typically located before the main clause, the genitive absolute “usually functions adverbially, and especially temporally, telling you when one thing was happening in relation to something else” (“Participles: Genitive Absolute). Thus, the “if” in Heb 10:26 is more of a “while” or a “when.”
In other words, Hebrews 10:36 is not describing the relationship between a condition, which might be hypothetical, and a resulting consequence but between two events that are temporally related to one another (compare to genitive absolutes in Mt 25:10 and Lk 17:12). Because the participle in Heb 10:26 is in the present tense, Heb 10:26 is comparable to a present general conditional statement (“Greek Conditional Sentences”). The present general condition is one that indicates “a generic situation or universal truth” (compare to conditional statements in Jn 11:9 and 2 Tim 2:5).
In my opinion, because both the genitive absolute and the main verb are in the present tense, the genitive absolute is similar to the English zero condition.
“Conditional Sentences” (madonna.edu)
The zero conditional uses the present tense in both clauses and is
used to talk about something that is always or generally true. The
present tense signifies that these actions are both possible and
typical.
Form: If + simple present, simple present
Example 1: If it rains, I take an umbrella with me to work.
Example 2: If I wake up early, I always read in bed.
Context and a closer look at the verbs
But if we accept the above interpretation, how does the thought in Hebrews 10:26 impact the efficacy of Christ’s sacrifice? In Hebrews 10, there is an interplay between certitude and uncertainty, a dynamic that is played out in the parsing of the verbs – the aorist tense and indicative mood of the actions related to Christ, and the present tense and subjunctive mood of those related to believers. On the one hand, the aorist indicative signifies that the work pertaining to Christ is completed, once for all time (vv. 12, 14, & 20). On the other, the present tense indicates that the work pertaining to believers is still in progress (vv. 14 & 25). The subjunctive, “used to portray a probable or desired action,” adds to the sense of uncertainty (vv. 22-25) (“Mood Subjunctive”).
Hebrews 10:14 ESV (parentheses added)
For by a single offering he has perfected (aorist indicative) for all
time those who are being sanctified (present participle) .
Hebrews 10:22 YLT
may we draw (present subjunctive) near with a true heart
Hebrews 10: 23 YLT
may we hold fast (present subjunctive) the unwavering profession of
the hope
John 17:3 and conclusions
Following the same grammatical pattern, the parsing in John 17:3 of the verb portraying the action of men is also in the present subjunctive.
John 17:3 YLT
and this is (present tense indicative) the life age-during, that they
may know (present subjunctive) Thee, the only true God, and him whom
Thou didst send (aorist indicative) -- Jesus Christ
Christ’s offering is enduring, but whether it remains for each of us is a matter that involves the human will. I believe this is the meaning of “wilfully sinning” in Hebrews 10:26 YLT. It’s not just about the act or state of sinning, but about whether we will that which is against what God desires. Only when we desire to do the will of God does there remain a sacrifice for sin; when such is the case, God’s grace is there to help us (cf. Rom 7:18-22), and His mercy embraces us even when we fail.