First of all, Weapon Focus tends to add about 5% to your damage, which can be a lot. The problem for a ninja is that you would have to marry a weapon, which means marrying a tactic, and feats are in short supply. And bows are stupid for a ninja; a fighter can be a double threat, and a rogue can get by on defenese and flanking if forced out of their comfort zone.
Let's go back to TWF. It works with both light melee weapons (correct choice) and shuriken (also the correct choice). You can force sudden strike by being invisible, by feinting, by forcing your opponents to make Balance checks, and by attacking paralyzed or stunned opponents. Should this occur, you can make a full attack. You can use melee weapons when adjacent. At a range, you can make full attacks, with iteratives, using TWF, if you use shuriken. TWF is correct. Reach + AoOs is almost correct.
Everything else is incorrect. Bows are incorrect. Spring attack is incorrect. Why does an invisible tumbling character need Spring Attack anyway? Crossbows are incorrect, and unarmed strikes are incorrect. Using a one-handed weapon in the main hand is also incorrect, because you do not wish to invest in feats for two kinds of melee weapon. If you meet something that isn't sneakattackable, you fight TWF with short swords, or you fight two-handed with a mornginstar.
(Just in case this isn't clear, I am being tongue-in-cheek about the word "incorrect." Even though it's true.

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Human ninja (Correct)
1 Two weapon fighting, Quick Draw
3 Improved Initiative
6 Point Blank Shot
9 Precise Shot
12 Improved Two-Weapon Fighting
15 Improved Precise Shot