In all other situations, Steve would have agreed with Verity. If magic hadn't been involved or if he hadn't known Lucas. Verity forgot for a reason all right, because Lucas had wanted her to. "You forgot because Lucas made you. That's why your memory is spotty." Steve didn't actually have any proof Verity's memory wasn't perfect - but he assumed. If he was right and her memory faded in and out, she'd been Obliviated. Realistically, there was no other explanation. A mother didn't just forget that she had a kid. What Lucas had done was horrible, but it wasn't something easy to forget.
The request to wash up left Steve in shock. She wanted to wash up now? "Uh, yeah, sure." His voice held obvious tones of confusion. Didn't she realize what he had to say was important? He didn't exactly have all night here - Steve had no idea when Rabastan would be back, and he wasn't sure what, if anything, the house elf would tell it's master. "Scream," he said suddenly. "Scream like, like I'm hurting you." Steve didn't offer an explanation, but his words came out rushed. If Verity screamed, then the house elf would stay away. It would assume Steve was doing what he was supposed to be doing. If they were quiet for too long, the elf might come wondering.