Conversation:

"if the female became available for mating with another male immediately, this latter's sperm might replace the sperm of the first mate."

This is incorrect. The female can mate again, either with the same dog, or another one - the runt of the litter is usually younger than the others as they are from a later mating, and it's possible to have a litter with pups from two different fathers.

The reason for the tie is because males have quite a long ejaculation process! And it's not all about the bitch clamping down either, the dog's penis swells once it's inside.