OK here is a bit on the methods I use and it is because the sheet rock here is the most unreliable wall board I have ever seen and I have personally with my wife built 6 houses and modified several others. I am not a contractor but I am a fairly good Engineer. In our unit, one time with an anchor and it may or may not work due to the flimsy sheet rock of very low density. Suggestons.
1. You mentioned that there are previous supports, so fill those holes with putty and and paint the walls.
2. Do not put an anchor in anywhere close to a previous anchor spot.
3. I use these: The ones with the expanding butterfly legs on the back that spring out.
OR
4. I use a piece of one by 2 finished wood strip, I drill holes in the board, then put it against the wall and then punch a hole with an awl at each hole, that gives a good hole pattern on the wall. Then insert the spiral anchors, the ones with the
very heavy acme looking thread, I put a thin drop of good Gorilla glue on the anchor and when you screw it in, if distributes the glue.
5. Then put a thin coat of the glue on the wood and use a couple of screws to hold the strip to the wall and let it set overnight.
6. Depending on the anchor it may take a #6 or #8 screw, I use short ones to hold the wood on wall while the glue is drying, and the longer ones later, I also put a thin amount of oil on the screws, this minimizes the friction while driving the screw and will not turn the anchor. I found the sheet rock is so cheap, it may allow the anchor to turn, so the lube oil helps, you do not need a lot of thread engagement in the mounting screw, about .75 to 1, inch is enough. You can experiment with the screw in the anchor while it is not installed, hold it with channel locks to see how it goes, you can also pre drill the inside of the anchor to minimize the friction because that friction will tend to turn the anchor while driving the screw. But you do need a good solid thread engagement. You can bond the 1 by 2 in different directions to support the brackets etc.
