Small note, but doesn't Scoundrel give you PBS? That should clear up one of your feats.
Edit: Actually, that could do a good bit.
Assuming that step 1 is more important then step two, I'd say delay Assassin back. You can hit MAM 1 at level 8, which will give you all you need to drop your opponent down to 0, which means that, at level 8, you can at least do the sneak up, disguise, throat punch. You'll just have to wait a round to do the actual kill
.
In other words, levels 5-8 would change to this:
5. Soldier 4 - Teras-Kasi Training
6. Soldier 5 - Melee Defense, Stunning Strike
7. Scoundrel 2 - Martial Arts II
8. Martial Arts Master - Tae-Jitsu Expertise
If you want to get Fatal Hit in earlier and don't mind a little delay:
5. Soldier 4 - Fatal Hit
6. Soldier 5 - Teras-Kasi Training, Stunning Strike
7. Scoundrel 2 - Melee Defense
8. Soldier - Martial Arts II
9. Martial Arts Master - Tae-Jitsu Expertise