OK, if I take this advise… (as I wouldn’t know how to submit the JSON to REST), and add the generic mqtt things… how do I have to define them?
There is no auto-config, because these are some Arduinos.
… and what do I put into the item.
As an example, here are a few items sending / receiving mqtt commands:
String ZevaUNO_MAC "MAC address [%s]" <network> (gZevaStatus) {mqtt="<[mymosquitto:ArgyleCourt/Shed/Tower/Zeva16/MAC:state:default]"}
String ZevaUNO_Message "SysMsg: [%s]" <settings> (gZevaStatus) {mqtt="<[mymosquitto:ArgyleCourt/Shed/Tower/Zeva16/Notification:state:default]", expire="12m,Zeva Controller broken"}
String ZevaCellSetRaw "Cell sets raw [%s]" (gZevaStatus) {mqtt="<[mymosquitto:ArgyleCourt/Shed/Tower/Zeva16/Voltages:state:default]"}
Number Irrigation1_1_ValveVolume "Stn 1.1: Outer East Volume [%d l]" <sprinkler> (gIrri1All, gPumpStation1_ZoneVolumes_Chart, gPersist_rrd4j) {mqtt="<[mymosquitto:ArgyleCourt/Property/PumpStation1/Volume1:state:default]"}
Switch PumpStation1_SoilPermittivity "Stn 1 soil permittivity" (gIrri1All) {mqtt=">[mymosquitto:ArgyleCourt/Property/PumpStation1/Command:command:ON:soilOn],>[mymosquitto:ArgyleCourt/Property/PumpStation1/Command:command:OFF:soilOff]"}
You gace me examples for rules the other day:
// MQTT 2.x version
val mqttActions = getActions("mqtt","mqtt:broker:mymosquitto") // use Thing ID for broker Thing
mqttActions.publishMQTT(topicRoot + "Flow" + valveNumber, valveFlow.toString)
mqttActions.publishMQTT(TopicRoot + "Volume" + valveNumber, valveVolume.toString)
… as for “use Thing ID for broker Thing” I have (re)named to mymosquitto. 
[edit] I have started another thread: [Solved] Things: doing my head in :(
… and things (pun intended) have become much clearer by the minute.