It's pretty easy to start gaining. Adding as little as 500 calories can get you started. It won't be quick gains by itself but you'll see it start adding up. A candy bar and a soda will get you there.

The other thing you'll really want to consider is lowering your activity. If nothing else try to minimize how much you have to walk around your home. Like move furniture so it takes fewer steps to get a snack.

Finally you might want to embrace grazing. Make it easy to mindlessly snack. Introduce snack stations around the house. Grab a bag of chips when you go to watch tv. Make food easily accessible and you'll find yourself eating without thinking about it.

So I've always started with just trying to greatly increase my intake. If that doesn't work take about 2 weeks and limit your intake, make your body shift into famine mode and then try the first method again.
Sometimes when you're not getting any progress you need to back up and get a running start.

Definitely briefs. Boxers are too loose and boxer-briefs are too tight in the thighs. Also a big guy in small briefs is a classic look.

Depends on the setting.
Over for Business/Formal
Under for Casual

You're probably not going to pop the button on your jeans. They're designed so that isn't a failure point for most brands. Check out the construction of yours but all mine are solid metal so multiple layers of fabric have to fail before that button goes anywhere.

You want dress pants where the button is sewn on.

I've done as high as 10 in a week. So I guess it might be possible, but you'd spend the week painfully full.

So from my experience there are a few.
The first is the sneakiest because it's the point where you look around and go "I'm still average size, I don't need to diet." The will to lose starts slipping here.
About 50 pounds in you'll reach a point where even if you lose it all you never quite get back to peak condition and have picked up habits that make regaining the weight easy.
The first big one is about 200, it's either the point you look at the number and go "I'm on a diet starting now" or you accept that you're just fat now.
Following that every 50-100 pounds when the thought of dieting comes up and you'll think about it and go "Sure I could stand to lose a little," but each time that goal number isn't as low as it used to be. Sure the ideal would be to see 135 again, but you can't really imagine yourself going under say 225 anymore. Or was it 280? 330? It just creeps up as higher and higher weights start feeling normal.

Personally I replace underwear when it gets too tight to wear it how you like. Jeans and shorts when they get to the point they only fit under the belly. Tees as they start failing to reach the waistband of my pants (assuming the pants haven't failed the fit test). Sweats and pajama pants as they wear out, usually in the thighs or rear.

51, and only 2.5 pounds away from 52

I love that the dalgona coffee trend makes this easier to pass off. Buy a large instant coffee or drink mix and it doesn't look like a weird amount of cream, you're just stocking up for the week.