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So the data is ”injected” into a serial telemetry stream that only requires one (or possibly a few) links between the vehicle and the ground. This process of inserting all of this data in a specific pre-determined pattern is called commutation.

A more basic word is "multiplexing." If a bunch of separate data channels share a physical link, each coming across as if it had a link unto itself, you're typically either carving up the one physical link's frequency spectrum into channels (frequency-division multiplexing, FDM) or thinly time-slicing it (time-division multiplexing, TDM). I would suspect it's FDM in this case.

Not quite. All the telemetry monitors are "gathered" as digital words and injected into the telemetry stream one at a time. The problem is figuring out how to pull that stream of ones and zeros back apart on the ground. So every so often a known set of bits, specific pattern such as "fdc0f1_hex" (nonsense number I just made up)is inserted into the stream so the ground decommutator can "pull" the data back apart and "read" the individual words. This known "word" is what is synced up on and is called the frame sync.

We can further sub-divide this telemetry stream into sub frames with a counter or a sub-frame sync to allow greater flexibility in how the parameters will be downlinked. Lets say we are doing something special, like running the robotic arm on the Shuttle. The telemetry stream has a finite space, so using a different set of sub-frames, we can downlink more data at that moment on the arm than our normal State-of-Health (SOH) telemetry monitors.

Often this link is QPSK modulated allowing for a large data rate. I have seen data rates over 300 million (yes three hundred) bits per second.

Sorry for the multiple posts.

Note: before the radio frequency (RF) link is modulated with this digital bit stream, the commutation has already occurred. So all the RF link "sees" (and is modulated with) is a stream of continuous ones and zeros.