War tends to be a desperate venture where concern over any potential post-conflict sanction for misusing technology is far outweighed by the greater concern of losing the war. Winners tend to rewrite history to absolve themselves of whatever means they employed in their victory. The costs of losing often couldn't be any worse regardless of crimes against humanity committed during the fighting.
With the predictable failure of a landmine ban, the core issue should be eliminating the danger to civilians from landmines after the fighting has ended. A major problem with waiting to consider landmine removal until after a war is that quite often war is fought until the combatants have totally exhausted their resources, and are incapable of cleaning up their leftover landmines for years afterward.
There may be at least a partial solution to the long term problem of abandoned landmines in the way the problem of soft drink container littering has been addressed by charging deposits at the time of purchase. Conscientious citizens are encouraged to recycle their containers so that they can recover their deposit on the bottle or can. The adverse impacts on the environment caused by irresponsible consumers is at least partly addressed by attaching a cash value to the discarded container, creating a profit motive for others to collect and cash in abandoned containers. A similar approach might be viable in the landmine situation.
A removal deposit on each mine would be paid to perhaps the World Bank by all landmine manufacturers or purchasers. The cost of the deposit would become a normal part of the purchase price. The deposit could be recovered by the combatants on any mines that were "used" or recovered after the war. As an added benefit, the landmine deposit refunds could be a ready source of post-apocalypse recovery funds to start the rebuilding of war ravaged nations.
It would likely only be possible to collect the deposit on purpose-built mines, but that would probably cover the majority of mines used. A deposit slightly higher than the cost to remove the "deposit paid" mines could fund a "Landmine Cleanup Superfund" to pay for removing/destroying homemade and converted munitions mines.
Perhaps more importantly, the deposit on landmines would be like a bounty attracting private enterprise to compete to recover/disarm/destroy the now valuable commodity. There has never been a more effective way to turn a problem surplus into a scarcity than to create a profit driven demand. Mankind has hunted animals to extinction, moved whole mountains in search of a few shiny rocks, dug ditches across continents, braved incredible dangers, and solved intractable problems in the pursuit of profit. A sufficiently attractive landmine deposit could turn the battlefields of yesterday into the gold rush of tomorrow.
At present, the removal of landmines is generally considered to be the exclusive concern of the military. There are admittedly some things at which the military excels - especially those tasks requiring maximum energy and minimum finesse. The question of using landmines should undoubtedly remain a primarily military concern, but perhaps the removal of mines after the fighting is over might better be left to profit motivated specialists and technology.
I expect that with sufficient profit motive available, private enterprise could come up with ways to cost effectively deal with leftover mines using robotics and artificial intelligence. Nothing has ever spurred innovative solutions to seemingly intractable problems like a little long green. If you have a run of the mill fire, you call your local fire department. If you have a seemingly impossible to extinguish fire in a refinery or oil well, you call in the specialists. They may cost more by the hour, but specialists can safely do in minutes what would be impossible for average firefighters to do in an unlimited amount of time.
In every other area of life where profit motivated self interested private industry has been unleashed there has been a rapid development of new technologies and innovative solutions. Creating a profit driven industry in landmine removal will have the same effect. When removal technology becomes sufficiently advanced, landmines themselves may become an endangered weapons system that must be actively guarded to keep the removers from disabling active minefields before the fighting ends. The aggressive development and use of anti-mine technology offers the only meaningful hope for actually eliminating the use of landmines. A deposit on landmines would fund exactly that result.