"Do they all deny her? An they were sons of mine," etc.
The folio reads, "Do all they."
"From lowest place, whence virtuous things proceed."
So the folio reads and punctuates. The context shows that we should read when, with Thirlby. Two lines further on we have 'additions swell's,' which, reading 'swells,' is grammatically right; but the 2nd folio read 'addition,' and Malone, who is usually followed, swell.
"Where dust and damn'd oblivion is the tomb."
For 'damn'd' we might feel inclined to read dim, damp, or dank. "And in a dark and dankish vault," Com. of Err. v. 1; or (as in Tr. and Cr. iii. 2) blind. The text, however, is probably right, for 'damn'd' often merely meant what is odious, or hateful, was to be condemned or simply was reprehensible. "Surfeits, impostumes, grief, and damn'd despair."—Ven. and Adon. (See on Othel. i. 1.)