Had Paul ever visited Rome before writing Romans, and how was the Roman church established?
No, Paul had not yet visited Rome when he wrote Romans. The Roman congregation was established through the witness of other Gentile believers, not by Paul. Writing around AD 60, he expected to reach Rome later, about AD 63, and expressed his longing to visit and impart spiritual gifts.
Les teaches that when Paul wrote Romans he was still writing from another point in the Empire and had never been to Rome. The Roman congregation had been established by the witness of other Gentile believers, not by this apostle, and Paul recognized that. He wrote around AD 60 but did not get to Rome until about AD 63. Paul made mention of the Romans always in his prayers, longed to see them, and wanted to impart some spiritual gift. He had often purposed to come but was hindered, yet he remained a debtor both to Greeks and barbarians, wise and unwise, and was ready to preach the gospel to those in Rome.
For I long to see you, that I may impart unto you some spiritual gift, to the end ye may be established;