Hello All, I have a little knowledge here so here goes:
Topping is normally associated with Fir's and Leylandii, it will live but look ridiculous. These are not protected so I suggest you have it cut down, dug out and re-plant something deciduous and more manageable.
To kill any mature tree by topping/ pruning is extremely difficult especially when you imagine natural storm damage etc. Treating the cuts may cause toxic shock and cause more harm than good, most people believe creosote or any household paint is ideal but would you put that on a cut on your arm, they will heal naturally!!. Generally speaking only certain Maple, Chestnut, Oak, Laburnum ( basically furniture wood) or Mature Trees with preservation orders are protected but even reputable Arborists will turn a blind eye when all it involves is a little light pruning.
As for passing your neighbours back the cuttings, unless these are suitable for a table top or a sculpture I suggest you refrain for fear of being scrapped from their Christmas card list.