I wouldn't put any stock in the spelling of the quote from the "unspecified source". The record itself is credited to Day Costello.
Going through the scrapbook at the end of UM&DI indicates EC's dad used "Ross McManus" professionally in Liverpool (e.g., the Ross McManus Quintet), but switched the "Ross MacManus" when he moved to London in 1951. There's a lot of variation in his press clippings, owing mostly to sloppy copyediting I suspect. Not only are they pretty free with the spelling of his last name, they occasionally render his first name as "Russ" or even (once) "Bross". But in items where he presumably has more control -- letters to the editor or a letter introducing himself to the Joe Loss Fan Club -- he spells his last name "MacManus" at least after 1952 or so.
In UM&DI, EC says Ross added the "a" because it "looked posher and better in print", but also seems to say, more obliquely, that it also had to do with avoiding anti-Irish prejudice. In the context of the time and place, "posher" and "less Irish" might have largely amounted to the same thing. It makes sense that Ross would encounter, or fear that he would, more prejudice in London than in Liverpool. Liverpool being England's most Irish city, especially in those days.
And since no post by me would be complete without some borderline-creepy internet spelunking, I can also report that, by early 1952, Ross was using "MacManus" in important legal documents, such as his marriage certificate. A website called FreeBMD.com provides an archive of these kinds of records for England and Wales. We can see that a "Ronald P R MacManus" married someone whose surname was Ablett, in Bromley, in the first three months of 1952: https://www.freebmd.org.uk/cgi/information.pl?r=204575792:7489&d=bmd_1494280030&scan=1
. Likewise, a "Lillan A Ablett" married someone whose surname was MacManus around the same place and time: https://www.freebmd.org.uk/cgi/information.pl?r=204443854:9827&d=bmd_1494280030
Interesting that Ross's name is given as "Ronald P R MacManus". His name at birth was Ronald Patrick McManus. The presence of the second middle initial "R" indicated he was probably now incorporating "Ross" -- which was just a nickname he picked up as a young man -- into his legal name.
I can also confirm that EC was indeed "Declan P MacManus" at birth, as I found the closest thing were going to get to the actual (long-from
) birth certificate here: https://www.freebmd.org.uk/cgi/information.pl?r=209173885:8402&d=bmd_1494280030
(You can view a scan of the portfolio page entry for each of the links above with just a few more clicks. I was going to attach them, but the images are too big for this forum and would probably be illegible if compressed.)