This issue has been discussed in this forum several times before.
Most of these things are not liabilities at all. They are only promises made by politicians and expectations on the part of the public, and their value is based on the principle that present day unfunded promises will go on and on unfunded until infinity. Their value also depends not only on the discount rate being applied but also on what is put into the mix. I have seen much lower estimates.
Now, there is a law that says what cannot go forever it will eventually stop. In this case the promises will most likely be reneged as taxes are difficult to increase. In Britain and elsewhere health care is already being rationed.
Finally, one has to understand that right wing America's objective is the dismantling of the welfare state. Most of the alarmist talk about the deficit was in great part motivated by that desire.
On the dodgy accounting issue. The argument against marking at market is based on the fact that central banks are pursuing public interest and therefore are not profit seeking institutions. The only argument against this principle is in the form that a change to marketing at market would force central banks to manage their assets better by prompting quicker reactions on their part to changing market conditions.
