You will not be able to create them yourself, the build only gets up to 6th-level spells and Create Greater Undead (needed for Shadows) is an 8th-level spell. You will, however, get Command Undead (so any shadows you happen upon can be taken control of), Summon Undead V (so you can summon shadows if they'll help), and can use a scroll of Create Greater Undead without a UMD check (it's an easy CL check to use it, if memory serves).
As for sneak attacks, the Unseen Seer is used to add the Ranger spell Hunter's Eye (PHB II) to your spell list, and it even gives a +1 to your caster level with said spell. It also advances your sneak attack by 2d6, for a constant total of 5d6 with the Rogue levels. While Hunter's Eye is up, you eventually gain an additional 4d6 sneak attack. Over the course of the whole build, you're never far behind a pure rogue in terms of Sneak Attack while Hunter's Eye is up, and you also happen to gain a skirmish-like ability.
When it comes to opportunities to do sneak attacks, you also gain a lot. You will be able to cast spells like Grease to force enemies to balance, Glitterdust to blind enemies, get spells like Invisibility, Blur, and Blink as concealment options, and eventually gain concealment while affected by Haste, each of them opening up opportunities to sneak attack. You can teleport with Dimension Door, with or without shadows, and can cast other spells like Darkness and Shadow Walk if you feel so inclined. You also get solid defenses in Mirror Image and Protection from Energy if you run into trouble and need to buy enough time to get to safety. You can even use a spell in Complete Scoundrel called Evacuation Rune if the trouble turns out to be too much.
Finally, you get spells that a typical Rogue is going to want as part of their job, anyway, such as Arcane Sight to spot traps, Knock to open locks, Rope Trick for it's general awesomeness, and Lesser Telepathic Bond for relaying scouting info to the party. There are also spells like Spymaster's Coin, Scry Location, and Chain of Eyes that let you scout remotely and then teleport to a safe location with the party.