1. Buy an open ended flight (many flights can have the return date rescheduled-speak to the airline about how longer notice needs to be given)
2. It shouldn't be a problem to book accommodation online which offers free cancellation and provide this address until. It might be easier to stay in the accommodation a night or 2 before moving to your partners.
The reason for the post is rather I'm aware of how long an 'adult' can stay on a tourist visa bur wondered if he could piggy back if he wished to stay for longer (1 or 2 years), without having to do the visa runs etc.