Finally got back into messing with this.
Thank you Tony, this was the issue. The extension 999[hostextension] does not show up as an extension within the Extensions module, nor does the main extension that it uses affect the settings applied to the 999["host" ext #]. I had to edit the extension within sip_additional.conf then reload asterisk. Not sure why the bria cloud solutions manager didn't just mimmick the settings of the linked extension.
Also, there seemed to be a typo. For that bria cloud solutions extension, trustrpid and sendrpid show up as sendpid=no and trustpid=yes for that extension only without the 'r'.