Make good of mock wall & fit new thick wall board

Just removed a old fitted wardrobe and discovered a flimsy mock wall on the side end of the wardrobe. New bespoke wardrobe being fitted in 10 days. 1. Need old 1915s plastered wall made good and filled up. This is the top section of the wall. 2. Need bottom section (size of half a standard room door) built up with strong thick board. This is to make a new flush wall. 3. Strong thick board to be purchased and cut to size by the tasker. 4. Top and new bottom section to be plastered/seamlessly smoothed out with the rest of the wall. This is because walls will be visible on the inside of the fitted wardrobe.