Which XML transformer and query is most appropriate to extract this XML?

First of all, the xml utf-16 encoding seems to give errors in my workbench (2021.2) so I had to strip that away with for example a stringreplacer.

You can use a XMLFlattener to create unexposed attributes containing the values you are looking for:

Elements to match: place

Attributes to Expose: place.place_id, gas_price{},gas_price{}.type

Then you can use a attributeCreator with 2 Conditional Values.

new attribute: Price Premium

Value: 3 Possible Values

Test Condition:

If @Value(gas_price{0}.type = premium Then value is gas_price{0}

If @Value(gas_price{1}.type = premium Then value is gas_price{1}

Else <No Action>

And repeat this for the gas price regular.

If you have locations with lots and lots more prices (diesel, lpg, electric, mega-diesel, liquid-nitrogen, green electric etc. etc.) a listsearcher could save a lot of test cases.

You could use an XMLXQueryExtractor, if your fragment is in an attribute called xml

let $p :=fme:get-xml-attribute("xml")/place
for $g in $p/gas_price
return (
fme:set-attribute("place", $p/@place_id/string()),
fme:set-attribute($g/@type/string(), $g/text())
)

XML Input = None

You would then just need to expose attributes place, premium and regular

let $p :=fme:get-xml-attribute("xml")/place
for $g in $p/gas_price
return (
fme:set-attribute("place", $p/@place_id/string()),
fme:set-attribute($g/@type/string(), $g/text())
)

This returned a result, but only if I selected Single value as the return value, and said yes to the XML Header. 

In the end, the result only ended up being the XML version information.

<?xml version="1.0" encoding="UTF-8" ?>

Any idea why I might be getting the result this way?

So this worked, but only partially. See the results here. ID 11699 returns the price and the type of fuel correctly. But look at row 11703. It has prices, but the flattener doesn't seem to find the price. just the ID.

It seems like 11699 only has a single fuel type per XML fragment. Any idea how I might query these '2 prices in one result' records?

I forgot to expose the attributes. Rookie mistake! Thank you for the help.