whoami root - mount ... error: must be root. - What did I do?

[dcatoffm]

Basically, what did I mess up and how do I fix it?

# set
ANDROID_ASSETS=/system/app
ANDROID_BOOTLOGO=1
ANDROID_DATA=/data
ANDROID_PROPERTY_WORKSPACE=8,0
ANDROID_ROOT=/system
ANDROID_SOCKET_zygote=9
ANDROID_STORAGE=/storage
ASEC_MOUNTPOINT=/mnt/asec
BASHPID=5780
BOOTCLASSPATH=/system/framework/core.jar:/system/framework/conscrypt.jar:/system/framework/okhttp.jar:/system/framework/core-junit.jar:/system/framework/bouncycastle.jar:/system/framework/ext.jar:/system/framework/framework.jar:/system/framework/framework2.jar:/system/framework/telephony-common.jar:/system/framework/voip-common.jar:/system/framework/mms-common.jar:/system/framework/android.policy.jar:/system/framework/services.jar:/system/framework/apache-xml.jar:/system/framework/webviewchromium.jar:/system/framework/mediatek-common.jar:/system/framework/mediatek-framework.jar:/system/framework/CustomProperties.jar:/system/framework/mediatek-telephony-common.jar:/system/framework/mediatek-tablet.jar
BOOTCLASSPATH_EX=/system/framework/mediatek-op.jar
COLUMNS
EMULATED_STORAGE_SOURCE=/mnt/shell/emulated
EMULATED_STORAGE_TARGET=/storage/emulated
EPOCHREALTIME=1495427565.723985
EXTERNAL_STORAGE=/storage/emulated/legacy
HOME=/data/data/jackpal.androidterm/app_HOME
HOSTNAME=M10X
IFS=$' \t\n'
KSHEGID=0
KSHGID=10074
KSHUID=10074
KSH_VERSION='@(#)MIRBSD KSH R43 2013/02/19'
LINES
LOOP_MOUNTPOINT=/mnt/obb
MKSH=/system/bin/sh
OPTIND=1
PATH=/system/bin:/system/xbin
PGRP=5780
PIPESTATUS[0]=0
PPID=5641
PS1='# '
PS2='> '
PS3='#? '
PS4='  '
PWD=/
RANDOM=1662
SECONDS=84
SHELL=/system/bin/sh
TERM=screen
TMOUT=0
USBOTG_STORAGE=/storage/usbotg
USER=root
USER_ID=0
_=set
# whoami
root
# mount -o remount,rw /system
mount: you must be root
#

A little background, this is a dragon touch tablet. I rooted with kingroot but quickly found it both inadequate and invasive, so I set off to replace kingroot with SuperSU. After the second time fail, I tried a basic trick: copied mksh to another binary, chown root, and chmod 7755. After removing kingroot, the plan was to terminal with this root shell, manually copy SuperSU's su binary in place, and once working, remove this gaping security hole I made just for this purpose. Well, I got the root hash and I'm identified as root, but quickly discovered /system was mounted ro, so the obvious next move was to remount rw, which I was promptly slapped in the face with above. Can I get around this without spending another half hour rerooting with kingroot (again)?

 

[dcatoffm]

Nobody has any ideas?

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