98 Outback DOHC Timing problem

The direction you rotated to get back to the mark should not matter as long as you got there with nothing binding up. This is an interference engine and can get bound up and break/bend valves by hitting pistons. Starting and stalling must be a separate problem. The fact that it starts for a second tells me your timing is probably okay...nothing crunched. Look for a wire you bumped loose while doing the belt. Last resort you may have to take a second look for timing off a cog or two. Check for a YouTube video...you may have missed something during reassembly.

I should add that once the belt was on, I started it before assembling the rest (crank pulley, drive belts, covers etc). Everything electrical is plugged in, I have triple checked. I've worked on cars professionally for several years

And it doesn't act like a battery issue at all. All I can think is if I got the wrong belt and it's off a few teeth