Sounds to me like you need to take it to Nokia Care.

There is little more to do other than flashing the device, and, since it was done correctly, that's that.

Sounds like the SoC is shot, but I can't tell like that. Solid orange is good (if it's orange. If it's red, it's bad). Use a multimeter to check battery - any voltage above 3.6 is OK in the sense that battery is still there and kicking. 4.2 is full, 3.6 is empty.

Don't think it's the battery, if it were it would die not get stuck. I had something similar to your issue in the sense of it getting stuck about where yours was, but I solved that via flashing.

If you connect the device to USB while off, does it still power up and act like an USB stick? Did you try to boot it without SIM and SD?

ETA: I remember someone was talking about publishing an image for N900 EN with Backupmenu and Bootmenu already in - that could allow you to power up N900 with kbd open and check the filesystem - maybe determine if the rootfs chip is shot (this could be it - flashing doesn't skip bad sectors and a bad on rootfs would give you similar symptoms).