In theory, as deflation continues and mining blocks becomes more efficient, the transaction cost will approach .00000001BTC/ 1 Satoshi and will stop multiple times on its way down at the point slightly above where profitability reaches cost. It is also possible that the transaction fee will approach zero.
The reason this happens is competition will drive the price down to the point that only the most efficient miners will be in business. The price reduction is further amplified by the deflation in Bitcoin. We will find the exact stopping point from the most efficient miners who are willing to take the lowest profit margin.
This will also be a floating point because as transaction volume increases so will profitability allowing miners to further reduce their fee.
To further answer some of your questions:
1)Are there any estimates or more concrete calculations about that?
A: None that I can find. My prediction is that transaction fees will have a multi-tier approach where the most efficient miners will offer "dual" processing fees one of which would be 0BTC. The miners would split their processing power(a percentage based on profitability and a simple maximization problem) and include only transactions that met the highest payment tier would be included.
For example, if you include a 0 Satoshi fee you will only get 5% total processing power. If you include a 1 Satoshi fee you will get 10% processing power.
2)Is it even possible to foresee, as the "degree of network security" is a rather nebulous incentive for most (casual) users?
A: The degree of network security will be improved if users provide their own processing power but, this will not be necessary as mining will be profitable to the most efficient miners.
3) Will they thus be enforced by the software?
A: Fees will be enforced by miners not including transaction with too low of a fee into the block chain. This will cause a significant delay in the speed of your transaction. In the worst case scenario, where EVERYONE on the network requires a fee, you will have to mine your own block to get your transaction processed.
4)Will these fees better be absolute or relative to the amount of a single transfer?
A: A miner can choose to accept a transaction and include it into a block based upon any criteria that they see fit. It will be interesting to see how miners choose to tier their pricing.