That's not a plot hole and Hela never lost access to the Bifrost. Gungnir, the spear of Odin which Hela has in her possession throughout Ragnarok can also open the Bifrost (Loki does as such in the first Thor movie), and Hela no doubt knows that seeing as she is Odin's daughter and was his right hand for centuries before her imprisonment. The point isn't that Hela needs Heimdall's sword to open the Bifrost, it's that she can't have a rebellious faction running around behind her back with that very powerful sword. She's simply cleaning house before she moves on with her plans to conquer other realms, as any half competent ruler would.
Anyway, to answer the actual question of this thread, Valkyrie would win against Drax easily. She's not just your average Asgardian, she was once one of the most elite warriors of Asgard and has the crazy strength and fighting skills and ability to match. She even managed to survive against Hela for more than five seconds which proves how strong and durable she is. She'd pound Drax into the ground.
