Best way to normalise Lightroom Photomerge?

This is an extremely wide angle and with straightened horizontals the result will be massively stretched in the corners. The final result will look something like a butterfly.

That's problematic because it means extreme upsampling around the edges. It will only be sharp at the very center.

If it's not possible to move the camera further away, consider a narrower angle.

What few people realize is that stitching is optically equivalent to using a wider angle lens. The net result is exactly the same and you run into the same problems with straight lines/corner stretching. Beyond a certain angle, the stretching becomes impossible to handle and you need to resort to the fisheye view.

To get a decent result here, do it in Photoshop. Reduce the size of the center frame and put it at the bottom of the stack so that the others align to that. This avoids excessive upsampling, so you at least get a reasonably sharp result.

An alternative is to open the image in Photoshop and use the Adaptive Wide Angle filter to manually straighten those lines.

This depends on the projection you choose. Providing that you do not have a very wide panorama, you will get straight lines if you choose the 'Perspective' projection. There are limits to this kind of projection however.