HTS221is the humidity sensor of ST not us, so i can't give any comment on this part.
I think you can directly use BME280 which has the humidity and pressure output directly.
The dust and water directly into sensor hole will damage the sensor, so in your water proof design, you need to consider to use membrane.
You can refer to the HSMI document for BME280 for system integration guidance.
For your other questions, maybe you can give me some more detail information what you are looking for.
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