Yes, I believe it should be a hard "d" on the end of halberd. But I've heard the first syllable as both "hal" (like "pal") and the word "hall". I think for hauberk and haubergeon, either pronunciation for the first syllable is fine. Or at least, I've never seen anyone be picky about it.
The funny thing with those hard "T" sounds is that so many people believe they should be pronounced like a soft French-like "T" that they'll try to correct
you. But they're clearly meant to be hard. For instance, there's a period writing somewhere in which a play on words is intended, comparing a Sallet to a Salad. The similarity isn't there if you pronounce the helmet as "Sallay". I don't remember whose writing it is in, perhaps Das Bill will remember, since he's the one who told me about it.