How can I cover a mirrored wall with slats of mirrored trim?

Those mirrors provide a spectacular view reflection and a great amount of light. I am going to suggest an interim idea before you fully remove the mirrors. Have you thought of building a picture frame for them with the vertical pieces covering the seams between the mirror panels. You can buy some pretty great weathered trim pieces at Home Depot that you could dry brush with chalk furniture paint for a more weathered beach look.

As to removing the mirrors it depends on how they are attached. If they are glued to the drywall you will need to remove both drywall and mirrors. Then replace the drywall, texture and repaint, then apply your wallpaper. If the mirrors are only held in place with mirror clips, have someone hold the mirror carefully (protective gloves and glasses and tape a large X corner to corner). Unscrew carefully starting at the top, then to the bottom. Mirror removal is potentially dangerous- look for some online videos to see best practices.

I second the answer above!

I also agree with Beth W. and Beth H.

You can bust them up with a hammer - wear thick gloves and remove. Most likely they are glued to the wall.

thanks for your input - I wonder if I can find something chunky enough to make an impact? Those "slats" where the mirrors come together stick out quite a bit

If you have a table saw... use a "dado" blade to cut a channel in the wood to fit over the "slats".... BUT... I think that removing the mirrors would be the best safety and long term solution... replace them with bead board (to cover any glue residue issues) or wainscoting. then you could fill in the area above the couch with "beachy" designed or decorated mirrors in multiple shapes to continue to reflect light and the view. ....