This link contains loads of commentaries written by people who can understand the syntax of the original language
https://biblehub.com/commentaries/acts/28-15.htm#
I looked through several, all the ones I looked through said that the brothers came from Rome.
Two examples:
"And from thence, when the brethren heard of us . . .—Better, the brethren having heard about us. The seven days at Puteoli had given ample time for the news of the Apostle’s arrival to reach the disciples at Rome."
"And from thence, when the brethren—of Rome"
That same link above also talks about the practice of going to meet someone important in the first commentary
"They came to meet us.—The practice of going some miles from the city to meet one whom men delighted to honour was a common one. So the Jews of Rome had gone out to meet the Pseudo-Alexander who claimed to be a son of Herod (Jos. Ant. xvii. 12, § 1). So the Romans had poured forth to meet Germanicus (Sueton. Calig. c. 4) when he lived, and to do honour to his remains after his death (Tacit. Ann. iii. 5). So in earlier days, Cicero had been welcomed on his return from exile, journeying from Brundusium on the self-same Appian Way on which St. Paul was now travelling, senate and people alike going forth to meet him (Cic. pro Sext. 63, in Pison. 22)."
I think that Paul never wrote a letter to the church in Jerusalem because he was mostly an apostle to the gentiles.
I think if you look at the context of the passage in Romans 15, you'll see that he is intending to go through Rome just to get helped on his way to Spain.
Romans 15: 18 For I will not dare to speak of any things except those which Christ worked through me, for the obedience of the Gentiles, by word and deed, 19 in the power of signs and wonders, in the power of God's Spirit; so that from Jerusalem, and around as far as to Illyricum, I have fully preached the Good News of Christ; 20 yes, making it my aim to preach the Good News, not where Christ was already named, that I might not build on another's foundation. 21 But, as it is written, "They will see, to whom no news of him came. They who haven't heard will understand." 22 Therefore also I was hindered these many times from coming to you, 23 but now, no longer having any place in these regions, and having these many years a longing to come to you, 24 whenever I journey to Spain, I will come to you. For I hope to see you on my journey, and to be helped on my way there by you, if first I may enjoy your company for a while. 25 But now, I say, I am going to Jerusalem, serving the saints.
Notice that in all of Romans, Paul doesn't talk about seeing the Romans again, like he does to other churches
2 Corinthians 12: 20 For I am afraid that by any means, when I come, I might find you not the way I want to, and that I might be found by you as you don't desire; that by any means there would be strife, jealousy, outbursts of anger, factions, slander, whisperings, proud thoughts, riots; 21 that again when I come my God would humble me before you, and I would mourn for many of those who have sinned before now, and not repented of the uncleanness and sexual immorality and lustfulness which they committed.
I don't think writing a letter would count as building on another man's foundation. And it sounds like he's talking about visiting them mostly for fellowship, and for them to help him on his way to Spain.
That there were Christians in Rome before Paul got there and Acts 28 doesn't necessarily mean that Peter was there.