Isn't that a kind of thing you just have to bite the bullet and deal with directly, because waiting only makes dealing with it more expensive with time?
Just evacuate the people and blow it up, accepting that it'll inconvenience a lot of folks and the national budget even more, but will also briefly revitalize the local economy, as every contractor there is will maximize the opportunity to suck out the funds set aside for repairing buildings damaged in the explosion - and make sure to record the disposal itself from every possible angle, with most expensive cameras government can buy, as selling clips of it to the media industry would be a licensing goldmine and recoup some of the costs.
Hell, turn it into a spectator event, and tourists coming to see it may just pay for the whole thing.
If they can spend 5 millions just for mast removal, I am pretty sure they could build some rough reinforced concrete dome / something similar all around it (15m depth ain't something impossible to build on) that could absorb most of the blast.
Say 50 million budget. Of course red tape, bureaucracy and just laziness/incompetence of state bureaucrats would make such budget balloon and drag for a decade or two, but it should be doable.
Yes, but why be the politician or civil servant responsible for dealing with it when it will most assuredly be someone else's problem in a few years? The political upside of getting it done is low, and the risk and cost are high.