Strength Bonuses

When using Razor Fiend, the half-strength-bonus for damage applies to every attack you make while using it.

Razor Fiend and Two Weapon Fighting

The extra attacks (and attack penalties) from Razor Fiend and Two Weapon Fighting stack. Note that razor fiend only adds to total attacks, not attacks per hand.

So if you have 4 attacks normally on a full attack with a single weapon, enough Two Weapon Fighting feats to get 3 off-hand attacks, and all three 'extra attack' razor fiend feats, then you'd get 10 attacks on a full attack action (4+3+3), all at a -8 penalty (-6 from razor fiend, -2 from TWF).