Law and grace cannot be mixed in regard to salvation. Those two things are mutually exclusive. But living under the law does not mean that those who live under it were saved by works of the law plus faith.
Peter lived under the law and according to his own words his salvation was on the principle of grace. And according to the passage which you quoted Christ is the end of law for righteousness to everyone who believes. That would include all believers, even those who lived under the law. That is why we read this:
"Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all" (Ro.4:16).
At Romans 3:21-24 Paul speaks of the "gospel of grace" and he says that the righteousness of God apart from law was witnessed by the law and the prophets--v.21 (the OT Scriptures). Then in the next chapters he uses David, who lived under the law, as an example of a man "unto whom God imputes righteousness apart from works" (Ro.4:6-8).
The very first miracle mixes fresh ("living") water with the water of the purification rites of the Yhudim, (the six stone cisterns were about two-thirds full and were not emptied of their contents before they were filled to the brim), and that is the Good Wine according to the passage. So, yes, the two are mixed in the very first miracle performed by Messiah, (John 2:1-11, see also Luke 5:38-39). The Torah is holy, good, and spiritual, (Rom 7:12, 14), and is our tutor or schoolmaster to bring to Messiah, (Gal 3:24). It is therefore likened to an immersion, that is, an immersion into death, just as the immersion into Messiah is an immersion into his death, (Rom 6:3), for when the Torah is used "lawfully", as Paul says in 1Tim 1:8, one uses it against himself or herself to cut off sin, (Rom 7:25). There are two categories of Torah interspersed within the whole, Horeb and Sinai, which in the physical are the same mountain; but in the supernal way the one is of above just as Jerusalem of above, (that is Horeb), and the other is of below, (that is Sinai). The Torah of Elohim is the Torah of the mind, and is Horeb, and of above: the Torah of sin, (and death), is Sinai and of below. And "using the law lawfully" we may employ the Torah of sin, (Sinai), to cut off sin from ourselves, our households, (house-body-temple analogy), and our "members which are upon the Land", as Paul says, "Mortify the deeds of the body", and again, "Mortify your members which are upon the earth", that is to say, put them "to sleep", or to death, meaning to neutralize those "unruly members" of your house-body-temple because sin dwells in them. So then with the mind I myself serve the Torah of Elohim, (Horeb and of above), but against the flesh the Torah of sin, (Sinai and of below), Rom 7:25.
Moreover, I say, That the heir, as long he is a babe, (in the milk of the Word), differs nothing from a servant, though he be (destined to be) master of all: but is under executive guardians (Torah with Kings and Priests, (Psalms)) and house-stewards (Prophets and Writings) until the appointed time of the Father, (Gal 4:1-2).