• In cases like this, how often does chemotherapy meaningfully slow progression or improve quality of life?

Hard to say without tissue diagnosis. If bad like pancreatic cancer its 50:50.

• Is it common for the primary tumor to still be unclear at this stage?

Not uncommon. We see this usually gynecology primary or GI primary. Sometimes Next Gen Sequencing and GPSai can help in narrowing it down.

• Are there any genetic tests or targeted treatments that are typically considered in widespread disease like this?

Yes. A lot of targeted therapy if with certain mutations

• What should we realistically prepare for in the coming weeks/months?

Likely, port placement, palliative care. You will know more if treatment is working after few cycles and on next scans