In fact, you can run any form of European legal entity from any country. I.e., I can create an spółka z ograniczoną odpowiedzialnością (sp. z o.o.) in Poland, but run the business in Germany. It would be complicated and stupid, but legal.
You can! A foreign EU company just needs to get a "Betriebsnummer" (company number) from "Bundesagentur für Arbeit", which doesn't even require a Betriebsstätte (permanent business establishment, a branch that is not legally independent) or a "Zweigniederlassung" (branch office, legally semi-independent, but still part of the same entity) - and certainly not a subsidiary that has it's own legal persona.
SAP is basically the core of the German compliance machine. Most of the time, people get onto SAP not because its good, but there's a bunch of compliance regs, which basically say 'use SAP'. Noncompliance results in firms basically not doing business with you.
You could try to be boneheaded and comply with whatever standards they need your own way, but that would mean your business partners would need to do more due diligence and expose themselves to risk of what happens if regulators are not happy with the way you conduct your business. So you use SAP.
Is this based on actual experience? Because at a place I worked in the past we did business with BMW, Allianz, Siemens, Munich Re and others and never had to use SAP. Maybe it depends on what part of the delivery chain you are.
For instance with Munich Re you have to "pass" their compliance gate which is comprehensive but still has a lot of leeway.
Origin: German, HQ: German, Accounting regime: German, Main stock listing: German, Executive board: 5/6 German