In the US, are inside loops permitted while flying a US- registered DG-1000S sailplane in the "utility" category?
Stuff you quoted in your question seems to provide the answer you're after:

2.6 Approved manoeuvres Category „Utility“: The glider is certified for normal gliding in the &quot;Utility&quot; category. Simple aerobatics are approved but only without waterballast and with the weight of the rear pilot compensated by ballast in the ballast box in the fin see section 6.8.7.
The following aerobatic manoeuvres are approved with all spans:
...
Inside loop