Yep. Can you wait (with your brother or any other strong & willing people) until K opens the door to go to work, then get a steel-capped boot very firmly in the door and push inside? Or, get someone she won't recognise on sight to ring the doorbell - ideally wearing a hi vis jacket, holding a box or a clipboard so as to look like you're there in an official capacity. If she just calls out "who's there" try "Miss K? I've got a delivery for you..." insist it needs a signature or whatever, to get her to open up. Then, place foot very firmly and prepare to only stay still or push forward. Don't actually damage anything or kick the door down, unless you're prepared to fix and pay for it there and then.

See programmes about the work of bailiffs to watch how quickly and firmly it's done - in the UK "Can't pay? We'll take it away" is good for this, if it's available online.

Re: locksmith - can't you just get browse to bring an electric drill? Drill the lock, then once inside get it replaced and issue K and landlord a key. But only do this if K is out, else she could bolt the door from the inside.