From my experience effects are longer lasting than 4 weeks, you'd have to stop for 1 to 3 years, you'd still be at better baseline if you haven't done it at all but you'll be able to notice it at mental level. Of course smoking, not moving at all covid style, drinking, overeating and trashing your sleeping pattern will all help with depression if you want to achieve it faster.
no way thats true, especially for strength training. it takes about 2 weeks of not training to feel like a packet of jelly. at least if you're like me and sit on the computer most of the day
100%. Strength training is one of those things I've come to think of as body maintenance more than actually an athletic performance thing. Believe it to be equally if not more important than cardio