The fact that no early MSS copies have been found that bear Paul's name is a simple fact but this in itself is not sufficient evidence against Pauline authorship. For the scribes who copied the MSS, their eloquence or the lack thereof simply was not an issue. They simply copied what was before them one letter at a time. Paul's supposed lack of eloquence is simply an argument used by many who argue against Pauline authorship. This argument does not present Paul's statement in 1Cor 2:1 honestly. Paul never claimed to lack eloquence. He only told the Corinthian Church that he did not use eloquence as a drawing card for the gospel. There is absolutely no evidence of any kind either internal or external to suggest that the Hebrew letter was ever originally written in Hebrew. This is a popular speculation among some but is completely without historical or MSS evidence. The earliest extant MSS of Hebrews comes from the p46 MSS which possibly dates prior to the third century, certainly not later than 200 AD (third century) which is considered very early indeed although a fragment of Hebrews 11 was found that is said to date to the second century. I do not remember when the first copy of the letter appeared in the Hebrew language. I am sure you can probably find out with a little web searching. There would have been no logical reason for writing this particular letter in Hebrew since everyone spoke Greek as a common language. Personally, I do not believe Paul to be the author of the letter because of 2:3-4. Who ever wrote this letter was not an apostle.
Yes, Paul was an Apostle, but he also had visions from the Lord, and so he had heard from the Lord. But the closes scribe that was with Paul, was Luke; but then there were other scribes that were around. The verse that you have posted sound as if someone had branched out on their own and started writing to others, like the ones he referred as Hebrew. Paul's task was to preach to the Gentiles, not to the Hebrews.
Galatians 2:7 On the contrary, they recognized that I had been entrusted with the task of preaching the gospel to the uncircumcised, just as Peter had been to the circumcised.