You can use Seresto for Large dogs.

All Seresto collars are made in Germany.

If you don't open remove the original packaging, each collar has a shelf life of 5 years. Buying 2 or 3 at one time is alright as long as you use all of them within 5 years.

Do you mean Advantage Multi? It typically safe to use with a heartworm preventive but Advantage Multi also treats fleas, so it is a double application. We recommend that you speak to your vet first before placing any orders.

This is 27.7' (70 cm) collar. It should fit your dog easily.