It doesn't sound bad here. In fact, it sound like a different voice than usual (?!). It'll be more like a J if you go further north, as in Ulster. Further south (e.g., in Munster), it'll be a more subtle palatalization, though it should definitely still be distinct from D. Listen here:
(this is a different word beginning with deir since that website only has pronunciations for dictionary headings; deir falls under the heading of abair and so hasn't been covered).
Probably because that's not proper English. In English, you can say "Says you", but I'd think that has a different Irish translation. You have to remember that English and Irish use different sentence structures and you have to rearrange when translating. Irish is verb, subject, object (VSO) while English is subject, verb, object (SVO).
Would "Abair tú/sibh" be just as acceptable? I found this website that has two conjugations for every person: http://www.verbix.com/webverbix/go.php?D1=30&T1=abair
Nowadays, the present tense form of the verb abair is deir, and Abair is only used in the imperative, and there is no subject for the second person imperative - Abair é! means "Say it!" (when you are telling/ordering one person to say it, so the equivalent of "tú") and Abraígí é! means "Say it!" (when you are telling/ordering a group of people to say it, the equivalent of "sibh").
Abair tú/sibh may have been used historically, but it is isn't used any more.