Hello Ludvig, and please excuse the delay answering this :)

Re lignotuber, at that plant age/size it is visible in most specimens, but not always looks like a very prominent spheric swelling in all of them. No matter what, the dormant buds (what makes new shoots) are there.

I have tried root pruning of several intensities, from just dead roots to almost everything. If you prune roots it is advisable to prune the aerial part too, or bear with dessication risk (which does not mean it will die, it is just the plant is doing the task for you!). Even if you keep the plug well irrigated, transpiration of a full crown is higher than moisture captured by severed roots. Especially by summer.

I have managed to get root plugs with a depth of 40 cm reduced to 15 cm depth via root pruning transplanted and later obtained healthy plants from them. But always "starting over" the aerial part.

A warning here, in my experience, once root pruning takes place, planting an Eucalyptus out and letting it grow as a tree is also a risk for tree stability in the long term, especially in windy areas. The best way to obtain large trees planted out is by planting them as small tubestock with "full roots", or as a bit larger plant grown in large enough pots ("potted up") as to not get rootbound when planted out.

About sand/grit, I know some eucalypts can grow on it (they were planted on very sandy soils to immobilise dunes in some areas of the world for instance, and I have seen them growing on very thin rocky soils too). But I do not think it increases cold hardiness.

Hope this helps? :) I should learn a better Swedish XD