I'm a little confused about what your Alcan 3/C cable is. Is it 3-wire including ground, or 3-wire with ground also?
I gather it's #12 aluminum which doesn't bother me if you make a point not to repeat the mistakes of the 1970s:
- use terminals properly rated for aluminum, such as the Alumiconn (for 1 AL to 1-2 Cu) or ILSCO Mac Block (1 AL to 1-4 Cu). Both of those have bodies made of aluminum. When the lug body is aluminum, it plays well with copper because thermal expansion differences work favorably. Purple wire nuts Do Not Work, they are UL Listed but they fail a lot. Probably because of ...
- torque the wires to spec with a torque driver (this discovery wasn't even connected with aluminum wires; it was found to cause failures with small copper wires but obviously it fills in the picture of what happened in the 1970s with aluminum).
You cannot blow off ground, especially not in an outdoor kitchen. RMC or IMC (but not EMT) metal conduit underground qualifies as a ground conductor. A ground rod does not! Because that would rely on dirt to carry return current, and it cannot carry enough current to trip the breaker. Instant trip on a 15A breaker requires between 90A and 150A of current, and that's not gonna happen through dirt.
Further, if your cable has a green wire, it cannot be used for anything but ground. Thus, to run a multi-wire branch circuit or MWBC (2 hots 1 neutral) you will need separate black, red and white insulated wires.
GFCI's compare the current on hot(s) vs the current on neutral. They must be equal and opposite (sum to zero if polarity is accounted for). So as you researched, you can't share neutral downstream of a GFCI, as that would throw off neutral. You must use a 2-pole GFCI breaker.
Or you can use a plain breaker and multiple GFCI receptacles.
The benefits to GFCI at the source are: #1 it will protect the enroute cabling from ground faults. And #2 it will keep the GFCI indoors and out of the weather, which is better for GFCI longevity.
On a MWBC, it's your call whether to use a 2-pole plain breaker or two singles, however if you use 2 singles they must be tied with a factory handle-tie not a nail. Having them have independent throws is a code violation and would endanger maintainers.