They fit in without any problems. and you have to stir them when heating them up anyway. 👍🏻

Depends on how adventurous you are 😄

With the Philips Avent bottle warmer, you can always put Holle jars directly into it - this usually works without any problems. BUT:
The jars often don't warm up completely evenly because the appliance is actually optimised for bottles (i.e. liquid).

In concrete terms:
👉 Works without decanting - convenient and absolutely ok
👉 But stir well afterwards and check the temperature, otherwise you will have lukewarm at the top and lava at the bottom

If you want it perfect (e.g. for sensitive babies): decant briefly or at least stir in between.

In short:
Lazy = pure glass 👍
Perfectionist = stir / decant 😄