GuiXT version in SAP GUI patch level: Does the most recent SAP GUI patch level always contain the latest GuiXT version?

Not necessarily.
When Synactive releases a new GuiXT version it is made available on our download page and at the same time sent to SAP to be included into the next SAP GUI patch. Normally the standrad release procedure at SAP will take a couple of weeks. So it is not uncommon to find a more recent GuiXT version on our homepage than on the SAP GUI patch level in the SAP marketplace.
Since we always keep the versions backward compatible you can, if you want, combine a newer GuiXT version with older SAP GUI patch levels.