Cosmetic fix for a cracked wooden window sill

If this were my wall: I'd pay the $30 to have an engineer remove the towel rail and fit service valves to the pipework. Then you can take it off, replace it, or what-have-you whenever you want without worrying about the upstairs neighbour. Mention to the plumber that you'll be re-plastering and re-tiling so the service valve is properly proud of the wall.

Then, with the towel rail off, get someone in to plaster it. (Don't try plastering yourself unless you do it for a living, it's much harder than the fellow you'll get in will make it look.)

Then tile away. A smaller tile means you can generally get away with really quite big imperfections, a wall that's a couple of inches off true at the top should be fine. (See here for more on that.)

I'm guessing these shattered because of shock or perhaps from being fully extended more than normal since the front wheel takes a lot of the weight when drawer is fully extended. The rail is acting as a lever where 10kg in the front of the drawer could easily be the equivalent of 100kg on that wheel when the drawer is fully open. The other thoughts are if someone where to lean on the drawer or if the cabinet was being moved and dropped.

I've seen replacement wheels, but they tend to be for specific rails/sliders, and I doubt you'd get a good result without a perfect match. For replacing the entire slider, I'd look for a replacement with a high weight limit. I've seen sliders in the US that are designed for 50kg, which I'm guessing would more than handle your load.