If jim360 is right about humming the message, solving this puzzle could take you all day long :>)
I happened to solve 5 first, and since it could have been either type of clue (with two different rogue letters) I thought that other clues, or at least some of them, were going to be just as ambiguous, but perhaps it was unintentionally so. Presenting the clues in alphabetical order of the lemmas was a neat touch. And, unlike some others, I was amused by the denouement.
This reminded me of Apex's number 2394, "Mastermind" (9 September 1976). The across clues were all DLMs couched in the form of Mastermind questions, and in the preamble Apex recommended Chambers and "Jude's Obscure Encyclopaedia" as reference texts. Some solvers made enquiries of their local bookshops and libraries about where they could find a copy. As Apex later wrote in an apologetic letter to The Listener, "Apparently, Norwich was in a tizzy, Plymouth was ransacked, and harassed librarians telephoned THE LISTENER in a desperate and vain attempt to locate ‘Jude’s Obscure Encyclopaedia’. ... I did not expect anyone to take an ‘Obscure’ encyclopaedia seriously, but now, having established the need for such a volume, perhaps some enterprising publisher will see fit to, humorously, satisfy the demand." So, does anyone have a copy of Triple's "Lexicon of Bizarrerie"?
[I agree with the grumbles about KOHb. Had it not been for 4198, with its typographical peculiarity, as crosswhit99 points out, I would have got it wrong too (see the last sentence of my posting, number 43, for 4201). The Wikipedia entry that lists the foreign names of chess pieces prints the Russian knight with a lower case Roman b, and I suspect that that's where Sabre got it from.]
As to solving aids, as a scientist I would think it odd, not to mention foolish, if I didn't use every possible method at my disposal to solve a research problem ... But perhaps artificial problems are different.