Tower Unite

Before anything, you're gonna need a Jetpack. You can buy one from Rob's Imports.

Go inside the main building, the one you enter right after leaving the subway. You'll see a left wall and a right wall, sorta like pillars. Go to the right side of the right wall, and simply fly upwards - past the lights. There are gaps big enough to fit a player. The vent should be right in front of you. Hit E to enter the Dev HQ, and the achievement should pop.

Go to the plaza then pause the game. Select appearance, and hit "Skeleton." If you have a workshop model equipped, hit "remove workshop player model" and the option should appear.

To my knowledge, Gold Catsacks only appear in Silver Catsacks. You can buy them from The Stray or get them from events like Treasure from Sea (if i recall) if you have a Metal Detector - also from the stray.

The chances of getting a Gold are pretty minimal, but not impossible. Took me around 30 - just incase you're considering buying it! xP

You aren't limited to using just the space bar - you're able to incorporate the mouse too. This allows you to drop apples at a much more consistent and stable rate (provided you have a sense of rhythm,) dropping 5 each time (mouse->space->mouse->space->mouse or vice versa) worked for me.

A viable strat for all of these is to go to the "Find Lobby" menu, and join a game with a map you haven't done at the very last level, or the second last if you know what you're doing. It might take some waiting, and it would be faster if a friend set it up for you, but i was able to do it in no time (over the course of a few days.)