Yikes, sorry to hear about the issues! Have you tried booting in target mode from an external hard drive/computer to see if either the drive or the drive-related components (cable) are the issue? Have you tried using an external display to absolutely rule out the video issue (no external video would mean there is a video issue)?
I have had a few Pros exhibiting a "slow booting" problem sort of like this, and the problem was resolved by replacing the DC-in/sound board. In most of these cases Firewire was also not working. Since a logic board replacement didn't fix it, the only other major component is the DC-in/sound board, so it might be a logical next step to replace that.
Good luck!