IC controlled triac dimmer design, zero crossing detection and inductor sizing
To answer your questions in turn, starting with your optocoupler questions:


You want an optocoupler with a bipolar (vs. unipolar) input: this means the input LEDs are in inverse parallel, allowing you to sense both halves of the AC waveform.
Provided the optocoupler output is clean, that's more than fine for generating a zero-cross interrupt -- you don't need to bother with switching the interrupt edge, though, as the interrupt pulse will look the same on both half-cycle-transitions.
With the bipolar input optocoupler, the Vf actually works in your favor, giving you a pulse centered on the actual zero-crossing assuming a symmetric mains waveform.


As to your noise questions:


You are talking about the distinction between "leading edge" and "trailing edge" dimming here.  Some loads (like ordinary incandescent bulbs) are happy with either, while others require one or the other; even for loads that can use either, trailing-edge is superior as it generates less EMI.  However, a TRIAC can't be turned off, or "commutated", by its gate -- it relies on the AC zero-cross to commutate.  So, all TRIAC dimmers are leading-edge by their nature -- trailing-edge or switchable-mode dimming requires the use of IGBT or MOSFET switches instead of TRIACs, which is much costlier.
RC networks are used for snubbing because mains inductors are big, chunky, and expensive; snubbing also is needed to protect the triac from transients causing accidental turn-ons and such.


Last but not least, to address your concern about snubber design, Fairchild Semiconductor AN-3008 provides a design procedure.  Fortunately, most lighting loads (save for MLV transformers and perhaps magnetic fluorescent ballasts) are not particularly stringent regarding snubbing, so a first-cut snubber should suffice. (Motors are far worse!)