1) I see transaction fees as being quite separate from demurrage. As you say, with demurrage there is no need to use transaction fees for ensuring currency stability or as a reward for miners. I don't understand why in the future currency holders would pay through monetary inflation, or how that would be an externality. Care to specify?
No. Now bitcoin miners are rewarded through inflation (new bitcoins are created with each block that go to the miner) and tx fees, In the future only tx fees will pay for the work of the miners.
With demurrage, fees are also allowed, but freicoin holders also pay miners, not only the people who want to move their coins fast.
The externality is the following: some users that gain from the bitcoin network won't pay anything for the miners work. Currently every holder is also paying indirectly the reward for the miner, since this new created coins make their own coins less valuable.
Merged mining seems a good idea.
I think users will accept freicoins, but I have some doubts about the ease of use for commerce with the software as it is now. It isn't to everyone's taste to be deal with bugs in transfers or complications with wallets. Things will have to get simpler from the end user standpoint, if the currency is going to have wide appeal.
Things will have to get simpler from the end user standpoint for bitcoin too. There's many people working on it.