Does Deuteronomy 18:20 apply today, that even if someone is right 90% of the time and fails once its all over? (not the put to death part, obviously)”
Short answer No. Two covenants:
One covenant (Old) that exposes, condemns & judges ALL sin
Deut 28:15-68 Find ALL the CURSES imposed for breaking old covenant laws.
Another covenant (New) that pardons/forgives ALL sin.
Rom 5:20 Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound:
(NOTE: Where OT law breaking sin is found in abundance, NT grace can be found in SUPER abundance. Rom 5:1-2 teaches how we access God's super abundant grace)
Gal 3:10 For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them.
(NOTE: Anyone today rejecting/ignoring Christ's sin payment & resurrection. Preaching/imposing old covenant laws, will be judged by them)
2 Corinthians 3:7 the ministration of death, written and engraven in stones, was glorious, so that the children of Israel could not stedfastly behold the face of Moses for the glory of his countenance; which glory was to be done away:
(NOTE: The Ex old covenant law, written on stone, ministered death & judgment. And was always intended to be replaced by a better (spiritual) life giving covenant)
Heb 10:16 This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them
(NOTE: The Lord says, I will write this new (spiritual) life giving covenant in their hearts & minds)
Matt 27:
50 Jesus, when he had cried again with a loud voice, he died;
51 And, behold, the veil of the temple was rent in twain from the top to the bottom; and the earth did quake, and the rocks rent;
(NOTE: Temple veil because of sin, separated man from God, is RIPPED in 1/2. Old covenant ENDS right here)
Gal 3:13 Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree:
(NOTE: They put a kingly like crown of thorns on Jesus head. He became a CURSE, in doing so, REDEEMED us from, Deut old covenant law curses)
Christ our High Priest (Ps 110:4, Heb 4:14, 5:6, 6:20, 7:15, 26, 26) of a New Better Covenant, written in sinless blood:
Heb 8
6 But now hath he obtained a more excellent ministry, by how much also he is the mediator of a better covenant, which was established upon better promises.
(NOTE: Christ obtained a more excellent ministry, he is the mediator of a better covenant, established on better promises)
7 For if that first covenant had been faultless, then should no place have been sought for the second.
(NOTE: If the old Deut law covenant was without fault. No new covenant would have been needed)
10 (B) The LORD said; I will put my laws into their mind, and write them in their hearts:
(NOTE: The LORD promised to write His Spiritual law on the believers heart)
13 In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.
(NOTE: The promised New Better Covenant began at Pentecost. When Christ baptized believers with His salvation sealing indwelling Holy Spirit)
can you show me anywhere Hal Lindsay says hes a prophet. Hal has even talk about the "prophet" 
