I'd expect not to get Platinum benefits if the booking is not in the name of the Platinum member. You could be lucky of course, though in general Germans will follow the rules about this. It did not work for me in the Netherlands.

BTW: I think it's risky to book in your partner's name and trying to check in one day earlier, I would definitely contact the hotel in advance when trying to do that. Though of course there can be an 'unexpected' delay in arrival