5 Months and still no carbonation

Hi everyone. So I brewed a Barleywine back in May to celebrate my now 3 week old daughter. As others have done, I want to open a bottle on each of her birthdays. the beer ended up @ an FG of 1.020 @11.7% ABV. I added 1/2 pack of dry yeast and 5oz of corn sugar at bottling time. I wanted to overcarbonate it slightly so the carbonation would last through the years. The bottles have been kept between 70 to 80 degrees. Its been 5 months now and the beer has no carbonation. When I opened one of the bottles 3 weeks ago I thought I heard the tinyest hiss. Sounded about as loud as a dust mite coughing. And the beer had no percievable carbonation.

I know many people have asked questions about beer not carbonating. But most of them have only waited a few weeks. I not sure what the best course of action is. Should I up end the bottles and get the yeast back in solution? Or would that risk oxygenating the beer? Or is all the yeast dead by now? Should I open all of the bottles and add fresh yeast? I want this beer to last for the next 20 years so I dont want to risk oxygenating it if I dont need to. Any advice would be appreciated.

12% beers are difficult to carbonate using the standard method. You pretty much need actively fermenting, alcohol-tolerant yeast like WLP 099 to get the job done. At this point, your options are limited. Making a healthy starter of WLP 099 and eye-dropping some into each bottle may be your best bet. Or kegging under a CO2 blanket, force-carbonating and beer-gunning the beer back into bottles.

Other options are more risky - Brett, enzymes, etc.

How would I get the beer into the keg without oxygenating it? Would there be any flavor or attenuation differences between WLP099 and using champagne yeast?

If the keg is filled (and periodically re-filled) with CO2 and you pour the bottles gently, you will minimize (but not altogether eliminate) O2 pickup.

I'll leave it to someone with better knowledge of wine yeasts to compare it to the high gravity beer yeast.