I found this 9Now aussie plugin installs drmhelper as a dependency:
https://github.com/glennguy/plugin.video.9now
9Now won't run for me because it is for Australia, and I am in the US but glennguy was kind to explain in their chat that one can go to 9Now's settings-> debugging, and run widevine update from there. So I did, and it did update. It did not resolve the issue with Amazon VOD: with the new version of Libreelec 18 from today (devel-20180117211132-#0117-g3...) on Rpi3 and with just updated widevine it still does not work:
05:20:17.526 T:1838150544 NOTICE: [Amazon VOD] Using inputstream.adaptive Version:999.2.1.13
05:20:17.527 T:1838150544 NOTICE: [Amazon VOD] Checking AudioTrack
05:20:17.563 T:1942599776 NOTICE: VideoPlayer::OpenFile:
https://d2lkq7nlcrdi7q.cloudfront.net/dm/2$pIfuAWqdhm_dmKurYOhHb-AAmHo~/bbf7/c09a/1899/4cb1-b6a5-81634b29de
af/706fcce9-1e1d-4c1b-8146-8ada1b14b39b_corrected.mpd
05:20:17.573 T:1493779344 NOTICE: Creating InputStream
05:20:17.728 T:1838150544 NOTICE: [Amazon VOD] Player Starting: 0.0/0.0
05:20:18.239 T:1493779344 ERROR: Interface_Filesystem::get_property_values - invalid data (addon='0x628a0228', file='0x6289f550')
05:20:19.099 T:1838150544 NOTICE: [Amazon VOD] Player Caching: 0.0/0.0
05:20:19.100 T:1838150544 NOTICE: [Amazon VOD] Player Resuming: 0.0/0.0
05:20:19.100 T:1838150544 NOTICE: [Amazon VOD] Codec: Samplerate:0 Channels:I()R(1)
05:20:19.704 T:1493779344 ERROR: AddOnLog: InputStream Adaptive: Unable to find license;hdcpEnforcementResolutionPixels in JSON string
05:20:19.704 T:1493779344 ERROR: AddOnLog: InputStream Adaptive: License update not successful (no keys)
05:20:19.704 T:1493779344 ERROR: AddOnLog: InputStream Adaptive: Initialize failed (SingleSampleDecrypter)
05:20:19.707 T:1493779344 ERROR: CVideoPlayer::OpenInputStream - error opening [
https://d2lkq7nlcrdi7q.cloudfront.net/dm/2$pIfuAWqdhm_dmKurYOhHb-AAmHo~/bbf7/c09a/1
899/4cb1-b6a5-81634b29deaf/706fcce9-1e1d-4c1b-8146-8ada1b14b39b_corrected.mpd]
05:20:19.707 T:1493779344 NOTICE: CVideoPlayer::OnExit()