Build 1 less room than is required for the next star, and then wait until you have about 3 times as much money as is needed for the next star. Then build the last room, and when you get your next star, no guests will complain about not having the new types of rooms, as you will have enough money to buy them right away. This will keep your ratings up. Also, building your elevator(s) a few floors higher than you need before you progress to the next star can help.
Are the "random" backup weapons actually random, or are they based on how strong your primary weapon is? Every time I use the Gauss Rifle, I get a Machine Pistol and 9MM Pistol as backup...
The first 8 achievements are self explanatory.
There's no excuse: after you "take poop", simply don't use the sink.
Good hygiene: Opposite of previous: do use the sink.
The Stash: level 08. In the third area, look for the only cabinet you can interact with.
Snump: level 11. In the last area, right after the laser, the bathroom is in the upper left.
Friend of every nerd: level 12. Hack all the computers outside the cells.
What's mine is mine: level 12. In the second area, hack the computer, and open the crates.
No nerd left behind: level 13. Keep the doctor following you the whole time.
The pacifist: level 15. Don’t trigger the alarm. You can't use any of your weapons. Just watch the guards' paths, and wall crawl under cameras.
Spare time soda: level 17. Use the soda machine near the stairs, AND complete the level.
A link to the past: level 18. Go to the bottom right of the area. Interact with the vent.
Hope this helps :D
After you beat the game, there should be an option to continue. Relentlessly deleting all of your progress since your last save is a bad surprise, especially since you can only save in cities.
There should be an option to flip the keys around, so A or 1 would be the highest, for people who actually play guitar and are used to their pinky hitting the higher notes.
GREAT game series, keep it going, but try and make more missions, and maybe money can actually do something? Such as buy a new "computer", with a new look or something. Can't wait for Hacker 4!