About moving ice bergs northwards to slow down all the melting a tiny bit. ( Clearing away navigation hazards is a side effect of this.)
Some background. The idea of towing ice bergs to provide a source for fresh water in dry climate zones was first proposed in the 1970s by Weeks and Campbell (1973) and Hult and Ostrander (1973). This sparked interest that resulted in several studies published in conferences (Husseiny, 1980; IGS, 1980). The focus of the work was manly on the wastage of ice during transport and of course the technical aspects of towing ice. The main issue, however, concerns the integrity of ice bergs during transport. The mechanics of ice bergs is still poorly understood and it is known that bergs gradually break up. How this is affected by ocean swells and other forces acting upon a berg under tow is still investigated (see a wealth of publications in particularly Journal of Glacology and Annals of Glaciology). The idea has, however, not been forgotten and more possible technical solutions are proposed (e.g. Fast Company. So while the financial aspect of the process may be calculable, the strength of ice bergs, and the forces acting upon them in the open sea and under tow requires more research before towing could become a reality.

On top of this are legal issues. Who owns the ice? and also how can towing ice berg across busy shipping lines be safe, particularly in the event of a break-up in mid-ocean.

To respond to your specific question on towing northward and anchoring: The towing in any direction is subject to the same issues as described above. Anchoring, the ice bergs will melt and degrade over time so that moorings will continually have to be renewed. Most ice bergs in the north will go through at least one cycle of being caught in seasonal sea ice but eventually will be set free to follow ocean currents in whatever direction they move locally. Since the ocean water will carry some heat, it will not matter where the ice berg is located, it will slowly melt. On top of that is the issue of disintegration since melting will change the shape of the berg and hence its stability and centre of gravity causing stresses within the block to alter. 

If one takes a longer-term perspective and ignores the practicalities (incl. costs), moving ice to a place where it melts slower would momentarily decrease the contribution but eventually one would reach a new equilibrium where the same rate of melt water will be contributed and the temporary benefit will be lost. So apart from being highly impractical from all aspects, the longer-term benefits will be nil.

References

Hult, J.L. and Ostrander, N.C., 1973. Antarctic icebergs as a global fresh water resource. Rand Corporation, Santa Monica. Report No. R-1255-NSF. 83 pp.

Husseiny, A.A. (ed.) 1978. Iceberg Utilization. Proceedings of the First International Conference, Ames, Iowa, 1977. Pergamon Press, New York. 759 pp.

IGS (International Glaciological Society), 1980. Proceedings of the Conference on the Use of Icebergs. Annals of Glaciology, 1 136 pp.

Weeks, W.F. and Campbell, W.J., 1973. Icebergs as a fresh-water source: an appraisal. Journal of Glaciology, 12, 65, 207-233.