First off you can have one MQTT-in node passing to a switch. Just use a topic of particle/#. If you don't know about using single level and multi-level wildcards in your topics, you should go read MQTT Topics, Wildcards, & Best Practices – MQTT Essentials: Part 5 - actually read the enrite series.
terminology: you mean a JSON string or a javascript object
I would work backwards. setup an inject node and an influx node and figure out how to store data in the influx database. Remember Influx is not like SQL. While there are similarities, there are differences and I'm trying to wrap my head around this too but this might help: InfluxDB schema design and data layout | InfluxDB OSS v1 Documentation
Once you can create a inject node with test data in the format you want/need that inserts successfully into the database, you can now work on formating the data from the sensor into that structure.
I hope this gives you some help.
Update - take a look at this thread: Changing a payload.obects field name
