Just a few suggestions, hard to know precisely the best approach, though:
Try a different stain: If the MinWax gel stain you're using isn't producing the desired results, consider trying a different type of stain, such as an oil-based or water-based stain or a different brand of gel stain.
Apply more coats: If the blotchiness is due to insufficient stain, try applying additional coats until you achieve the desired color. Be sure to let each coat dry completely before applying the next.
Sand in between coats: If the blotchiness is due to an uneven stain application, try sanding the surface lightly between coats to even out the finish.
Consider using a pre-stain sealer: A pre-stain sealer can help prevent blotching and promote an even application of stain on softwoods like pine.
Apply shellac: As you mentioned, applying a thin layer of shellac as a sealer before staining can help produce a more even and darker finish. When using shellac, it's important to follow the manufacturer's instructions carefully, as it can be brittle and prone to cracking if not applied correctly.