I want to bond zirconia. My lab says that it can be bonded with CoJet (ESPE/3M). Another lab says it can't be bonded, so just cement it. What do you recommend?

Zirconia can be bonded.  The adhesive Panavia F 2.0 (Kuraray) directly adheres to alumina1 and zirconia2, without the need for any special primer. (Similarly, Bistite 2 DC (Tokuyama) directly adheres.)  Additionally CoJet (and the lab version, Rocatec) can bond zirconia but it more expensive and most likely not as good as Panavia. SilJet from Danville is equivalent to CoJet.
For many years, silane has been used to bond to HF etched silica ceramics. Recently, clinicians have been confused about the use of silane on the newer, non-etchable ceramics such as alumina and zirconia. On such ceramics, silane does not yield a stable bond.3  In an attempt to eliminate confusion and create a more "universal" bonding protocol,  Clearfil Ceramic Primer (Kuraray) was introduced to bond both HF-etchable silica based ceramics and other non-etchable ceramics, such as alumina and zirconia.  This Primer combines silane for silica ceramics... It has been shown to enhance the CoJet bond to zirconia. .

I am having trouble with bonding incisal edges. I am using Prelude as directed and placing Estelite Sigma as my primary composite. I placed a small bevel (1mm) on the facial and lingual kavo surface margins and adjusted occlusion in CO and all excursive movements and it has failed both times. I would appreciate any tips you could recommend.

Your choice of materials seems fine. I trust that you used Prelude with the phosphoric etch option, which is best for enamel (etch, wash, dry, #2 Prelude, dry, cure).  The probable reason for failure is the mismatched mechanical properties of enamel and composite. For that reason, I have recommended using porcelain or pressed ceramic on incisal edges, especially after one composite failure.  Here is an example of a porcelain incisal edge repair, viewed at 16 years recall.  Bonding of porcelain to porcelain was done with sandblast, Clearfil Porcelain Bond, flowable composite.

I am worried about color of a veneer that is thick and perhaps not fully light curable. What would be your recommendation for a resin cement in this situation? Would you consider the Light shade Panavia? If so, do you think I should a bonding agent like All Bond since there is so much dentin exposure? I would really appreciate your thoughts and recommendations. Please let me know if you need any more info.

Panavia's Light shade is not very white nor very opaque. (The adhesive monomer in Panavia is dark brown so it limits what shades can be produced.)  Better would be to use Accolade PV. One of the shades will have sufficient opacity. Determine the best shade using the Accolade PV Try-In Composite. I must stress the need to keep the majority of the opaquer in the composite, not in the veneer. The lab needs to know the final shade but you take care of the blockout by using the opaque shades of Accolade PV. If it is a very fine lab, you might ask for a "fog of opaquer" in the deepest part of the veneer. While All Bond 2 is a fine adhesive and a reasonable choice here, for a more optimal bond to the fill-in composite, use my secret weapon: Clearfil Activator added to the Photo Bond.  Sandblast with a Microetcher prior to application of the adhesive is ideal to optimize the bond strength. The Photo Bond is dual cured and does not require any light to cure. Photo Bond's catalyst ...

Is Gluma still the answer to elimination of sensitivity? There are clones, Microprime B and Microprime G, what is the best choice?

Gluma Desensitizer (Heraeus Kulzer) continues to work well with total-etch adhesives (those which use a separate phosphoric acid etching step). You may be interested to know that Danville's Microprime G is a clone of Gluma  (35% HEMA and 5% glutaraldehyde).  CRA's November 2007 Newsletter recognized that  Microprime G is priced about 80% less than Gluma.  Wow, what a savings.
The other version of Microprime, known as Microprime B. (Take a guess at who "B" is). This patented version uses benzethonium chloride in place of the glutaraldehyde. The trade off is less soft tissue burning with B but somewhat less disinfection than with G. In terms of sensitivity reduction, I think the performance is about the same for both versions. HEMA appears to be the desensitizing ingredient as shown by Japanese research: Suda et al., Dental Materials Journal, 9 (2) 163-166, 1990.

Can Clearfil Porcelain Bond Activator be used with Prelude SE?

Yes, Activator performs very well when used with Prelude SE. When using Clearfil products, the Activator is mixed with the Primer (such as SE Primer) but with Prelude SE, Activator is mixed with the #2 Adhesive. The resulting bond strength to sandblasted porcelain (no HF) is as good as with Clearfil SE.

How can I opaque metal in an endo opening through porcelain, to hide the access opening?

You need Accolade OP Mask (Danville Materials, now Zest/Danville), which is supplied in White or Tooth Color shades. Be sure to light cure it thoroughly since it is opaque. 
Most often you would Microetch the endo opening, apply a Clearfil product like Photo Bond mixed with Clearfil Porcelain Bond Activator or SE Primer mixed with the Activator. (Prelude SE #2 Adhesive may also be used with Clearfil Activator.)  After light curing, the Accolade OP Mask is applied and light cured, then some composite is placed.