Widevine L1 is supported, yes. Tested myself using the 5G variant. No idea about camera2api.
There is a newer chinese ROM released since end of November, V10.3.18.0.PEMCNXM. This includes the October 2019 security updates. Any chance that the xiaomi.eu_multi_MIMix35G_V10.3.17.0.PEMCNXM_v10-9...