You could try to add a "s:" in front of this variables.

That worked. And it's one that I will put into any script wrapper.

That’s worth remembering, thanks, I gave up trying to do this a while ago and worked around it another way/

Other things are still not clear [from the documentation alone]:
- auth_proxy, does it want udp: or sip: prefixes, or is it just host[:port] ?

Wants the sip: proxy

- expires, in seconds I presume ?

Yes

- flags, cannot find anything about that ?

- reg_delay, I suppose that delays registration, in seconds ?

Yes, delays the initial registration but it’s a minimum, sometimes it will be more

- socket, is it like udp:host[:port] or udp:ip[:port] or some other form ?

udp:ip:port

All this is unclear from the current documentation.

I wonder if you’re looking at an older version of the documentation, most of it is here https://kamailio.org/docs/modules/devel/modules/uac.html but some of the older documents had some bits lacking iirc.