The problem with the lose bricks is also one i am having with my house built in 1850 nothing is remotly square even tied in ? I would have thought a rough render would hold it altogether and give it strength then i would use a stud work frame with carefull measurments you should be able to...
Have you thought about using a smoke match to see if there is a down draught of air As taking down the pot can cause down draught problems . any bleed thro from the tar in the stack can cause problems of shadows etc .
hi how old is the house if its victorian then the render can be over 3cms thick then you are into old crumbling brick work so you will need a 6 cm screw just to hold a picture up
first i would take of all the plaster board to expose the stud work as it looks as if its wet or been wet check what could have caused it / and cure that next give it a coat of preservative to prevent any further problems .I cannot see how the wall could have bowed suspect the plaster board...