Putting the right frame in the middle of my panorama

Lightroom really cannot do this. There are very few controls in the panorama stitching tool. I use the outstanding (and free) open source tool hugin (https://hugin.sourceforge.io/) or PTgui (commercial pano stitching software based on the same code base) for this. Both require a steeper learning curve than the pano stitching in Lightroom and Photoshop but they at least allow for complete control. I do most of my 360 panos using hugin: https://goo.gl/maps/Xr5K41aJcTLUrp6o6

Just as a general FYI for future reference: this is very easy to control in Photoshop simply by putting the center frame at the bottom of the layer stack. That becomes the reference frame the others align to.

And don't use Photomerge! That's an automated "easy mode". Use Auto-Align > Auto-Blend.

Unfortunately, there's no equivalent to this in Lightroom Classic.

a 2-step approach gives you the possibility for a precision line-up:
1. make the panorama a seamless 360° image:

• create two guides on the right and left sides marking the exact identical features
• crop the pano horizontally to this guides

2. center the altar

• use a guide to mark the center of the altar
• use a 2nd guide to mark the center of the panorama
• draw a selection rectangle between these guides
• check the info panel for the pixel width of your selection
• go to filter > other > offset and enter the pixel as horizontal offset with the option wrap around enabled
• in your case, the value to enter should be negative as the altar is right of the center

this should precisely center the altar in your panorama

What you could try is create the panorama in two steps, so you first create two 180 degree pamoramas. If you include the photo of the altar in both 'half panoramas' and then stitch these together, then I would expect that the altar becomes the center because it's the overlapping part of the two images used to stitch the final 360 degree panorama. I never tried this however, so I have no idea if it really works.