1 - many of the bigger lenders have included an automatic three month extension because of lockdown. Some will do the same but on a discretionary basis.
2 - it doesn't really. Your offer expiring and purchase not completing won't impact your score at all. Any impact on a score is at the point of application, and is negligible unless you are doing loads of applications to loads of different lenders.
In reality if your offer expires and you still intend to complete you'll probably just reapply, and unless your finances have changed then they'll just say yes, like they did last time. And you'll have a whole new six months to play with.