Is that offer actually good?

Almost always. They've already decided they want you: that's the most leverage you will ever have with them, and asking is a normal part of hiring rather than cheek. Real exceptions are narrow: some public-sector and union roles sit on published scales, and some big companies use bands where base pay barely moves. Even then, ask.

They could. It almost never happens. And when it does it's not the asking, it's the how: an ultimatum, a number with no reasoning, or reopening something you'd already agreed. One polite counter with a reason behind it is routine, and most managers are expecting one.

Depends on your market far more than any rule of thumb, which is why a percentage from an article is a weak thing to build on. What makes an ask land is evidence: a real range for your title, level and city, and a reason you sit where you say. No data? Getting some beats picking a number.

Often more than the money, and it's where a no on base still becomes a better offer. Signing bonus, start date, holiday, remote days, title, equity, when your first review happens, training budget, relocation. Ask what the whole package looks like before you decide it's fixed.

Often no: in a lot of US states and cities it's illegal to even ask you, and several require the employer to post a range instead. Where asking is legal you can decline and talk about what you want for this role instead. Worth checking your own city, because this varies wildly.

Let them go first. Ask what the role is budgeted for. If you're pushed, give a range from real market data with your target near the bottom, because you'll rarely be offered more than the number you say out loud.

A few days, and almost nobody refuses. Get it in writing, then ask for a specific date. If someone hands you a same-day deadline on a permanent job, notice that, it says something about how they treat people.