Note that I'm not the one who is saying that land ownership is theft - you are, because you are trying so say "taking by (threat of) force is theft".I'm not saying that land ownership is theft. I don't advocate taking someone else's land by force, but I do claim that (at least in "proto-world") taking unclaimed land is acceptable exactly because it is unclaimed.
That whole discussion does lead to some other interesting points though... for example, if I abandon the home and garden I build on unclaimed land in proto-world, and move 8,000 miles away, does that land remain "mine" or does it return to its unclaimed state? It turns out that in the modern world, "we" decided that once you own a piece of land, it is yours forever, unless you sell it, trade it way, etc. I'm not actually convinced that is right. On this, I may be closer to the position of some left-anarchist types who hold that land ownership is temporary and only based in actual use/occupancy. I'm not entirely convinced one way or the other on this point though.
Inheritance is another interesting issue, although you can kinda sorta hand-wave around that by saying that "if you can transfer something to a new owner, then doing so a split second before you die is effectively the same thing as allowing inheritance".
In reality, what is and isn't theft is defined by social convention. So land ownership, police returning items, and tax aren't theft, even though they all involve (the threat of) force.
Sure, the social convention is what it is. What I'm arguing is that the social convention is wrong and needs to be updated. It's not like these things are chiseled into stone tablets and made immutable for all time.
I'll also point out that I'm far from alone on the "taxation is theft" thing. It may be a minority viewpoint, but it's hardly some fringe idea held by just two or three crackpots. There's at least five or six of us. :-)