It seems to me that you're using a rule of expression I don't necessarily see dictated here. I can't see any requirement that if we're not condemned by the law then we must therefore be condemned by grace. I certainly see the possibility of such expression, which is common enough but what if the emphasis is on what we are affected by (bad or good) rather than how it affects us? For law can, by nature, condemn, but grace never invokes such a thing. Couldn't "under the law" mean condemned by the "law of sin and death" spoken of in Romans 8:2, which simply refers to the fact that "the wages of sin is death" as stated in Romans 6:23?
Peter himself claims that Paul wrote many things that are hard to understand, and yet I notice that many people handle his writings as if any 5-year-old child could easily grasp them. I wouldn't confine Paul's sophisticated intellect to any type of expression. I have asked before if anyone would volunteer some passage from Pauline text that they find virtually impossible to understand and I never get any response. I wouldn't study Romans 7 and 8 together for years because there were so many interpretations of it circulating, none of which I could settle on or improve on.
I confess I don't have the virtually allergic reaction to the word "law" that many seem to have today. When I was a child "law" was something that kept people safe and free to live prosperous lives. The church I grew up in (Southern Baptist) held the Ten Commandments in the highest regard and kept Sunday significantly "holier" than it is today.

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1a. If
under in "under Law" means "condemned by", then that would render the phrase "condemned by Law"... but that would also force you to read "under Grace" as "condemned by Grace"--and, importantly, it wouldn't go very far in explaining why that arrangement would result in us not being "mastered by sin" (as that is what the verse is teaching).
1b.
If I am correct in defining "Law" as "the Law of Moses/God", then we would expect that someone who
IS "under Law"
WOULD be "mastered by sin". Isn't the unsaved Jew, depicted in Romans 7, who is "under Law", "mastered" by (ie, a "slave" to) sin?
Yes : among other things, he is
sold under slavery by sin.
Why?
"The Law came in that the transgression might abound", and "When we were in the flesh, the sinful desires,
aroused by the Law", and, as 1 Cor 15 says, "the Law is the strength of sin".
2. The "Law" spoken of (in "under Law") is
unquestionably the Law of Moses, because this formulation ("under the Law") is used elsewhere:
i. 1 Corinthians 9
19For though I am free from all
men, I have made myself a slave to all, so that I may win more.
20To the Jews I became as a Jew, so that I might win Jews; to those who are under the Law, as under the Law though not being myself under the Law, so that I might win those who are under the Law
Obviously, "those under the Law" are Jews--and they are "under the Law of Moses". The term "under" means "under the jurisdiction of"--the Law of Moses dictates how they are to live.
There are other examples of this formulation that could be given, but you get the point.
ii. The Pharisees of today (and Paul was a Pharisee), basing their beliefs on the ancient Talmudic traditions that were being formulated in Paul's days, and in days that came before his, actually espouse the same belief that "death frees a Jew from the Law" expounded by Paul in Romans 7:1-6
Niddah 61b
Our Rabbis taught: A garment in which kil'ayim4 was lost5 may not be sold to an idolater,6 nor may one make of it a packsaddle for an ass, but it may be made into7 a shroud for a corpse. R. Joseph observed: This8 implies that the commandments will be abolished in the Hereafter.9 Said Abaye (or as some say R. Dimi) to him: But did not R. Manni10 in the name of R. Jannai state, 'This8 was learnt only in regard to the time of the lamentations11 but for burial12 this is forbidden'?13 — The other replied: But was it not stated in connection with it, 'R. Johanan ruled: Even for burial'? And thereby R. Johanan followed his previously expressed view, for R. Johanan stated: 'What is the purport of the Scriptural text, Free14 among the dead?15 As soon as a man dies he is free from the commandments'.
Except for the fact that Paul bases his argument on Torah Law, it is essentially the same. He argues that the Jewish believers have died with Christ, and that that death has effected their freedom from the Law. "Through the Law, I died to the Law" Gal 2:19 means "The Law Itself demands my freedom from it--my freedom from the Law is totally legit. It's demanded by the Law." This is how he could affirm that he "walked orderly and kept Torah" without being a liar--he couldn't blow his cover in Jerusalem, the center of Jewish living, but, since those who walk after the Spirit keep its righteous requirements (Ro 8:4), and since his freedom from the Law IS "orderly" and "in accordance with the Law", he could affirm that without lying and without blowing his cover/destroying his opportunity to witness to the Jews.
It doesn't mean the Law is bad, and it doesn't mean sin is good, it just means there is a new means of serving God--ie, the Spirit of Grace. To the point, the Gentile believers are deemed "doers of the Law" without knowing the Law (Ro 2:13-15, 26, 27). by dint of this new method of service (the Spirit of Grace writing the Law on their hearts--the fulfillment of the New Covenant promise).
How ever anyone wants to work this doctrine out, the result must be holiness, taking care of one's neighbor, because those who have Grace fulfill the Law's righteous requirements.
3. The difference between "works of the Law" and "justified by works" (James) is that "works of the Law" are "a righteousness of my own", but "justified by works" refers to working God's works, abiding in Christ by walking in faith so that "God Is My Righteousness".
Paul, in Galatians, says the Galatians are wrong for being "under Law" (Gal 4:21)--he doesn't direct them to "Christ alone", he instructs them to "through love serve one another". Christ's Name is "God Is Our Righteousness". God is love. Faith works by love. Therefore, the person who is walking by faith the Spirit works in his heart is walking in love and God is his righteousness. He is a "doer of the Law", and is justified, but if he does what he doubts, he is "condemned" (Ro 14:23).
4. There are some issues with this explanation that I haven't gotten worked out, but I'm not uncertain about what I've said.
Paul does say, "Children, obey your parents
in the Lord which is the first command with a promise". What's the difference between
Paul issuing the command, and them reading it in the Law?
Paul says "in the Lord"--as elsewhere, he stipulates that one's life is to be lived "as unto the Lord", and we "abide in the Lord" by keeping the two commands to believe in His Name and to love one another (1 Jn 3:23, 24). We love others when we're living according to our convictions (Ro 1:17, 14:5, 23): "My righteous one will live by faith, but if my righteous One draws back, My soul will have no pleasure in him" ("without faith it is impossible to please God"). So, as we are abiding in the Lord, we are to obey our parents, which sounds like "Don't stop abiding in the Lord because of this, don't let your parents pull you away from the Lord with anything they say or direct you to do, but... don't be unaware that honoring your parents, while you're retaining your God-given convictions before the Lord, has a blessing attached to it."
So, yeah, that is a little "problematic", but it doesn't at all break the deal.