>I have also never heard during as derring. Daring is "derring".

this is probably just a matter of different pronouncing guides. how would you pronounce 'derr'? I wouldn't think with a long a. I would show daring as /DARE ing/ and during as /DUR ing/ (midwestern flat dur, with no j or y influence)

>By the way, why is derring-do always written that way, and not as daring-do?

interesting question -- it was given to us that way by Spenser and Sir Walter Scott (and glossed by them) as an erroneous pseudo-archaism.

c1374 Chaucer Troylus v. 837 Troylus was neuere vn-to no wight+in no degre secounde, In dorryng don [v. rr. duryng do, dorynge to do] žat longeth to a knyght+His herte ay wiž že firste and wiž že beste Stod paregal, to dorre don [v. rr. durre to do, dore don] that hym leste. 1430 Lydg. Chron. Troy ii. xvi. (MSS. Digby 232 lf. 56a/2; 230 lf. 81a/1), And parygal, of manhode and of dede, he [Troylus] was to any žat I can of rede, In dorryng [v. rr. doryng(e] do, this noble woržy wyght, Ffor to fulfille žat longež to a knyŠt, The secounde Ector+he called was. [edd. 1513, 1555 In derrynge do, this noble worthy wyght.] 1579 Spenser Sheph. Cal. Oct. 65 For ever who in derring doe were dreade, The loftie verse of hem was loved aye. [Gloss., In derring doe, in manhood and chevalrie.] Ibid. Dec. 43, I durst in derring do [mispr. to] compare With shepheards swayne. 1590 I F.Q. ii. iv. 42 Drad for his derring doe and bloody deed. 1596 Ibid. vi. v. 37 A man of mickle name, Renowned much in armes and derring doe. 1820 Scott Ivanhoe xxix, Singular+if there be two who can do a deed of such derring-do. [Note. Derring-do, desperate courage.]