Does this head strap fit in the official Meta protective case?

Yes, the size of this battery head strap is the same as the size of the official Meta Elite head strap, and it fits perfectly in the official Meta case.

Does the 15W output watts meet the charging requirements of the Quest 3?

Generally, a 15W charger should be powerful enough to keep the Quest 3 from discharging while it is being used, but this will depend on the specific application. For example, if you are playing a demanding game, the Quest 3 may consume more power than the charger can provide, and the battery may slowly discharge. However, for less demanding applications, such as watching videos or browsing the web, the Quest 3 should be able to maintain a charge while using a 15W charger.

Is it better to have a 15W head strap or a 20W/25W head strap and how do I choose?

We can't say that 20W is necessarily better than 15W, or that 15W is necessarily worse than 20W, as we said earlier, the 15W version can meet the charging requirements of Quest 3 most of the time, and at the same time, due to its lightweight, it can also maintain the comfort of long-term wear.
It mainly depends on what your use scenario and demand pursuit are, if you are a long-time VR player who plays for more than 6 hours continuously, then the 20W or even 25W version is suitable for you, if you are an ordinary VR player who plays less than 5 hours continuously, then the 15W version, which is more lightweight and comfortable to wear, may be more suitable for you.