It would be the bride and groom giving the actual details themselves, although I think it might be the official who married them who initially fills out the form for them to sign and take to the Registrar...someone else might be able to confirm who does what.
As to the husband knowing his mother-in-law's name...he may not have had his MC at hand and also, emotion can take over at a time like this and it wouldn't be unusual to forget details.
I was just looking at one of my DC's today where the informant is the deceased's son. He got his deceased father and grandfather's name correct, he said his mother's ms was Murray...it wasn't - it was Millar. He said his paternal grandmother's name was Christina _________, it wasn't...it was Catherine Alexander, but his maternal grandmother's name was Christina.
Regards,
Anne H