What a lot of confusingness.
1. Testing IR to find a leakage fault does not require L-N test, which in any case you should not do (and cannot get a meaningful result from) when loads are connected.
2. When testing to earth, the loads will usually link L & N together by a low resistance so the readings of L-E and N-E tests will be the same. Therefore, there is no point doing these two tests separately with loads connected.
3. Just in case the only parts of loads connected between L & N are not of low resistance and cannot stand 500V without damage, you should solidly connect L & N to prevent that any voltage appearing between them. Therefore, only one test can usefully and safely be done, which is from L & N linked, to E. This is how a PAT tester tests insulation.
4. Most devices can withstand 500V on this test, or put another way, if they can't they aren't truly safe. But any appliances or cables with surge protection will give a failure reading, usually something under one megohm, as the surge protection clamps the 500V test voltage. They will usually pass easily at 250V, so that is a good reason to use this voltage when doing a global test to localise a bad load device.
5. Tracing a fault on the installation itself, with all loads disconnected, can and should be done L-E and N-E separately at 500V.
Not all leakage is due to poor IR but it is a good place to start.