Thx for answer :-)
All the problems solved somehow after again freshly did firmware-installation (which creates fresh filesystems).
Seems my tests did somehow "corrupted" something (not in uci I checked but maybe in /tmp or or or).
At the end it looks like working as below:
WIFI_IF_NO="`uci get multi_wifi.@wifi-iface[-1].priority`";
uci set multi_wifi.@wifi-iface[-1].enabled='1';
uci set multi_wifi.@wifi-iface[-1].ssid='***';
uci set multi_wifi.@wifi-iface[-1].key='***';
uci set multi_wifi.@wifi-iface[-1].priority="$WIFI_PRIO";
When disabling/enabling/deleting MultiAP-Wifis, it must be checked whether this wifi has been recently used, then it is still set under "uci show wireless" and you might need to disable/enable/delete it there too (not only under multi_wifi).
As far as I tested the multi_wifi restart is only needed in case the wifi (which needs to be disabled/deleted/enabled) is already set under "uci show wireless" as the recently used one.
( Btw, the array-number -1 is only working well in case there is already array existing with at least one element, but not fully tested )
Many thx for all you help,