Your interpretation is (partially) wrong.
" But if they do, then it seems that (1)-(4) just are four or five characteristic substantive positions that left-libertarians tend to hold, and that non-left libertarians tend to neglect, ignore, or consciously reject"
<= This does not follow. Those ‘substantive’ positions do not affect the core of libertarianism. Let’s take Rothbard’s example definition. A libertarian is someone who wants the world based on the NAP principle. Well; left-libertarians can happily say that they want a world based on the NAp principle (like how Rothbard interpreted it). So they are libertarians. The term ‘left’ merely singles the possibility of any of those 1-4 points of view that do not affect the core of libertarianism as such. That is what they are signaling. That they are libertarians; and that libertarianism will have these kinds of effects.
These are four or five characteristics positions that left-libertarians tend to emphasize and that (they believe) non left libertarians tend to underemphasize.
That is not a difference in substance; just a difference in emphasis. Cause it doesn’t affect the core. Of course there is ‘some’ difference - what would there to be signaling about otherwise?
“and it’s hard for me to see how these specific things that left-libertarians believe in are something other than substantive differences over issues of social and political analysis.”
<= Try harder. It is. There is no real difference (usually) on the normative side or in the analytical framework. Thinking that (1) current social processes have certain outcomes rather than others or (2) thinking that the abolition of statist social processes wil lhave certain outcomes does not make one ‘not’ a libertarian or ‘different’ than other libertarians.
Some libertarians think that a libertarian world would be more economically unequal, with larger cooperations. Others think they will be more economically equal, with more (but fewer) cooperations. The second category consciously calls themself ‘left-libertarian’.
Of course; there need to be some difference between ‘regular’ libertarians and ‘left libertarians’, else there wouldn’t need to be a signaling mechanism (what would it signal if there is no difference?) But the difference is in style, analysis of impact of the totality of governement intervention and the consequences of the abolition of these. Those are just differences in opinion that are independent of the label of ‘libertarianism’ as such. One is not ‘more’ or ‘less’ a libertarian because one thinks a world based on the NAP (to give an example) will be ‘more’ or ‘less’ economically equal. Both are libertarians. But there is ‘a’ difference; and to signal this (possible) difference, some call themselves ‘left-libertarians’. But both want a world based on the NAP; so both are properly called libertarians.
To put it differently: the ‘left’ denotes a specific approach/point of view to issues that are contingent within the libertarian framework (not constitutive of ‘being a libertarian’). That is what it signals. It signals something, but it signals something that is not a change of the libertarian conclusions, but in the consequences a libertarian world will have.
If you want to call that a ‘substantive’ position; sure. But then you are using the term on a different level.
Edit: presuambly - after checking your bio - you selfdescribe as a left-libertarian? (The 'free market anti capitalism was a dead give away.)