Presumably a big part of the Scottish left not challenging the ISG is that a) a lot of it shares some of the same basic assumptions, eg the SSP and to an extent on independence and of course on many other things the SWP too; b) there are a lot of people around who just don't like to swim against the stream/rock the boat, in Scotland as elsewhere.
I don't think the ISG's line on Scottish nationalism is Leninist. The dispute between Lenin and Luxemburg was not about whether to idealise nationalism - which Lenin certainly didn't do and in fact argued sharply against. It was about whether to advocate the right of independence for oppressed nations in Europe eg Poland (as I understand it Luxemburg didn't disagree with advocating this right for clear cut developing world colonies). I think on this Lenin was right and Luxemburg was wrong - but I don't think this has much to do with the ISG's position, which is semi-Stalinist caricature of Leninism, in the long tradition of Stalinists idealising "good" nationalities and demonising "bad ones" (cf Israel/Palestine, which is much more straightforward than England/Scotland, because it actually is a situation of national oppression, but where such idealisation/demonisation is still wrong).
The point you make about being defined negatively, rather than positively by what you're for, is a crucial one about much of the left in Britain today. It's what allows the siding with various reactionary forces and regimes over the backs of workers, women and other oppressed groups in the name of anti-imperialism. (And I think the lack of clear positive principles is also part of what allows for general manoeuvring, demagogy and political dishonesty in how leftists relate to each other and to the broader movement.)