Walnut burl veneer, wiped poly into wood and wiped surface clean. This was done to help set off the grain.
After poly had dried (15 days or so), sprayed fresh shellac on.
Once shellac had cured for several weeks, began sanding the finish level. After a week or so, shellac began showing signs of crazing and wrinkling in some areas.
I would blame the poly, but it had thoroughly dried, and it was not built-up.

As notes:
I mixed up the shellac with some lacquer thinner added to control runs and blushing. Sprayed on with HVLP setup.

Any ideas as to the why's, possible fixes (without stripping and starting over)?

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What poly? Minwax? I'd say the shellac is more brittle than the poly, and sets up a barrier issue. Basically a hard finish on top of a flexible one. I'd wash it all down with denatured alcohol to remove the shellac, scuff sand, then add more poly. Next time, ditch the poly if you want to use shellac. If you want a warmer color, use darker shellac flakes.

I had never tried this before but I had a while back a fairly well-known refinisher's book, in which he stated that one could rub boiled linseed oil or tunge oil into the wood to pop the figure, before spraying with shellac or lacquer...I did assume that since there would be no surface build, that poly being a harder finish than either of the other two, would be an equal or better choice.

I reeeeally would hate to have to start over on this- but, if there is no other solution anyone has, it is what it is.
ANY ideas, experiences, or views on this method on using the oils would be appreciated.

Shellac and lacquer are both harder than poly. Don't confuse durability with hardness. You also have to consider how "hot" the solvents are. Mineral spirits is a lot milder than denatured alcohol and lacquer thinner.

my sentence wasn't clear I guess... I did not mean poly was harder than lacquer or shellac-- but rather harder than linseed oil or tunge oil...

So you are suggesting putting a harder finish on top of softer again? No matter what kind of oil you put on the wood, the surface will always have some kind of residue on top. Wiping will never remove 100% of it unless there is some solvent used. Even then very difficult to remove it all.

I am not attempting to promote anything.
Being still a relatively new refinisher (I admit I will have a lot to learn for a long time - I doubt I will ever be able to claim "I know") - I try to see a logical pattern.
As I said, a well known finisher mentioned in his book that rubbing BLO (boiled linseed oil) or tung oil into the wood before spraying with shellac or lacquer would help "pop" the figure of the wood (since both shellac and lacquer dry so much faster they do not seep in as deep).

I tried this replacing the oil with polyurethane (yes the fast drying minwax). In "my thinking" I considered that the poly would be 1) harder (than BLO or Tung- since it contains resins) and 2) would dry faster and a bit lighter.

Part of my logic was:
Cocobolo, Rosewood, etc., are oil woods (these natural oils do not cure at all). To prevent finish problems - shellac is commonly sprayed on to seal the wood and those oils away from a finish like polyurethane or such.
The finisher, I mentioned, suggested applying the BLO or tung (curing oils) and spraying shellac to seal this against the desired finish (i.e. basically making the wood an "oily" wood - except those cure).
I then assumed (progressing up the chain of curing - i.e. non-curing, curing, crosslinking (i.e. poly)) that poly would be just as good or better choice.

I posted my original question in the hopes someone might have a "silve bullet" to fix the problems I encountered. I too suspect the poly (but in reality I can not be absolutely positive). I should have prepared test pieces - as a finisher I used to converse with used to always stress. I guess what I am saying is - I don't want to just realize that I have to strip the old finish and start over (as revolting as it is - appears so), I want to understand the truth of why if failed (if the finisher and his blo/tung oil method works - why it didn't for me - was it really a large enough difference to switch to from his suggested oils to a poly).

Thank you for your previous, and future, input - I greatly appreciate it

Like Rich C said, you violated a cardinal rule of finishing by applying a harder finish over a softer one. This rule mostly applies to "film" finishes that are on top of the wood vs. penetrating finishes like tung or BLO. Crazing is exactly what you would expect to see in this situation. You may have luck removing the shellac with just denatured alcohol and some Scotchbrite. It may start out as gummy mess, but a few passes should get you down to the poly. Not sure what your solution is after that.

Good luck!

As the others are getting at, the suggestions that "well known finisher" is making are to use penetrating oils, not film forming ones.

Poly forms a film on top of the wood.
The oils in question soak into the wood and do not leave a film on top.

Oils that form films would be just as bad.

For most situations like this, you can find a set of alternative ways to accomplish your goal that are easier.
You don't need to use either poly or shellac, for example, you could probably just use some dyes to accomplish the same chatoyance depending on the wood.

(and them light amber tint or something in the topcoat to simulate warmth and depth)

Thanks for the replies.
I see my mistake was assuming I could get all the poly of the surface of the wood...Lesson learned, thanks for the info- much appreciated.

BTW, don't ever let it stop you from experimenting and trying stuff.

The only way a lot of know anything is from screwing it up a lot (sometimes on test pieces, sometimes on real ones :P), and hitting on ways that work, and trying to perfect them for our workflow.