Unfortunately, you have failed to turn anything on its head!
The SNP are free to campaign for election in England, Wales and Northern Ireland. There's no law that prohibits them, you know! But they, you and I know they won't get elected in England, Wales or Northern Ireland.
Scotland is NOT separate from the rest of the UK. If it were, your campaign for independence wouldn't make ANY sense whatsoever! So, any party from any part of the UK can stand in any other part of the UK. After all, England, Scotland, Wales and Northern Ireland are INTEGRAL parts of the ONE sovereign state aka country known as the United Kingdom of Great Britain and Northern Ireland. See above - the SNP can stand in ANY part of the UK. Not a problem!
Your attempted argument about French and German political parties standing in each other's respective sovereign territory is laughable!
That's because Germany and France are NOT part of a sovereign state called Germance! They are TWO separate, independent and sovereign states, you know.
Your attempted comparison falls flat on its face. Completely.
Scotland within the UK is far more comparable to Catalonia within Spain or Quebec within Canada. It's also comparable to Bavaria within Germany.
Yet Quebec, Catalonia and Bavaria have all been shown that they have zero democratic rights to secession from their respective parent states. You see, territorial integrity of sovereign states takes precedence.
The only way self-determination takes precedence over territorial integrity is in cases of oppression by the sovereign state or in a colonial context.
Evidence here:
"In a UN context, the right to self-determination in its external shape is applicable to people (not to national, ethnic, and religious minorities, whose rights are recognized in Article 27 of the ICCPR, 1966) or to the nations in the cases of: a) a colonial context; or b) in a situation of any foreign domination or occupation."
"Today, it may be concluded that international law bestows on all peoples the right to self-determination, but that the right to external self-determination, exercised through remedial secession, only applies in extreme circumstances, to colonized and severely persecuted peoples."
Fortunately, Northern Ireland, Scotland, Quebec, Catalonia and Bavaria are neither examples of colonialism NOR are they in any way oppressed by their parent states.
No wonder you wanted to compare Scotland to Germany and France! 🤷♂️
Therefore, your bogus comparison is what we call False Equivalence and falls flat on its face.