Nobody knows how tunnels work. Because there are no tunneling invaders. So there are no numbers. We're at the ideas stage with a vague brief of 'tunneling invaders night be fun'. Some people find coming up with ideas for games fun. You don't, great, we get that, by why purposefully derail a thread?
Hanging out here repeating yourself over and over again until the thread dies only causes people to assume your long walls of text on economics are just efforts to kill those threads too rather than anything actually worth reading. Heck, maybe that's true....
Coming up with ideas for the game is the suggestions forum, I was led to believe that general discussion was where this thread belonged because it was 'speculation' about a future feature to be implemented. So there is no ideas stage as such, since this is not about making suggestions as to how tunneling invaders would work but instead speculating about how we would in the future situation be able to counter a situation the mechanics of which we no nothing.
If I repeat myself it is because I got no response that addressed anything that I was saying and as for 'what people think', when did I ever care about that? I see no value in pandering to a 'cosy consensus' brought about by bloodthirsty crypto-fascists taking advantage of the general desire of ordinery people to avoid confrontation and this is pretty much how I recall the forum was when I arrived. Or to put it another way, the velvet glove concealing an iron fist.
It is pretty easy to speculate how digging invaders will work given there's already a plugin by the very same name that implements it. If tunneling invaders will work like in that plugin, where invaders will dig as little as they need to and not at all if possible, then you simply have to make sure that a (preferably trap-filled) path remains to your fortress at all times in order to protect against it.
The problem is that that if what was planned was that simple it would already be in the game. If invader digging is supposed to work like present digging works, then the absence of it for invaders does not make much sense. However, perhaps the reason for the absence is that it requires quite advanced AI to avoid basic pitfalls like you are describing? The AI must be coded according to the present numbers and mechanics, but what if the present numbers and mechanics *are* a placeholder.
Recall that the whole situation is going to upended when we implement multi-tile creatures.