Wolf, I think your dignity when you are alone is different than when you are with others. One always has to remember shivisi H’ lenegdi somid, but if it doesn’t denigrate your own dignity to eat on plain plastic plates, then you should feel free to do it in private. You don’t have to invite the office to come into your cubicle. Would you be embarrassed if a coworker stopped by and saw your plastic plates? A woman is permitted to be less tznius in her own home than she would be out on the street but when opening the door has to make herself presentable.
I think a person who goes with his jacket off his shoulders gives off an air of being casual, so if he is with CEOs and CFOs, then he could be insulting them. I agree with what Mod42 says about daavening. I’m not sure about the hat tipped back part, but untucked shirts and jackets off the shoulders is for sure causal attire for daavening and would be better off with less formal wear but properly worn to be respectful.
Maybe the true madreiga to strive for would be as you say that things would not matter if we were in public versus in private, but taking into account human nature, such madreigos are hard to maintain and without a true necessity to force a person to hold there I personally think it is okay to relax on these strictures when one is in private. You might think differently and that’s fine.