Is the Subaru 1235 F1 engine an actual boxer engine?

[coaster]

I could imagine main journals 2 and 4 being completely milled away on a cnc 3 axis, it sure would be nice to find some photos of that motors inner modifications.

[vorticism]

So it became a 180* V4 then? That's a rare one.

[coaster]

It would have a yaw rocking motion, but still balanced as a opposed mass still travels in the opposite direction.
The 12 cylinder would 3 sets of that crank configuration spaced at 120 degrees.
To keep forces in line with each other as much as possible.
1+2@0
3+4@180

5+6@120
7+8@300

9+10@240
11+12@60

[Tommy Cookers]

It would have a yaw rocking motion, but still balanced as a opposed mass still travels in the opposite direction.
The 12 cylinder would 3 sets of that crank configuration spaced at 120 degrees.
To keep forces in line with each other as much as possible.
1+2@0
3+4@180
5+6@120
7+8@300
9+10@240
11+12@60

IMO there is no merit in this case above ....

opposite crankpins aren't better (though yes they are in the Prodrive engine of course)
because eg 6 equispaced pins cover the good effects that the opposite-spaced pin approach has

all flat 12s (and flat 6s) ever made are 'mirrored' ie symmetrical end-to-end - and so have notionally perfect balance
but at best a flat 12 as above isn't symmetrical end-to-end and so has some moment imbalance aka rocking couple(s)
(moment imbalance meaning here that the inertia forces are balanced in magnitude but not balanced in position)
plus it has 2nd order force imbalance as these on the first 4 and last 4 crankpins don't cancel ...
plus the pin 5 & 6 cancels neither the 2nd order forces nor their moments ....
(plus the firing intervals aren't even and the inlet and exhaust layouts might not seem attractive)

unsurprisingly its rather like the F2 Tecno-style 4 pin 8 cylinder flat (aka 180 deg V) engine - quite a lot of vibration etc