try to run cmk --debug --detect-plugins=webmon_slot3 -vvII 835-VALD-WebMon command without the --detect-plugins=webmon_slot3 option and check if your detect section works.
About the detect section, if I am correct you are trying to match two OIDs the are not in your SNMP walk. Try to use sysObjectID or sysDescr to detect if your plugin should run. if these OIDs don’t work for your device, use an OID that you query in the plugin anyway.
detect = any_of(
equals(".1.3.6.1.4.1.994.3.4.7.17.1.68.3.0", "1"),
equals(".1.3.6.1.4.1.994.3.4.7.17.1.68.3", "1"),
),
here a sample detect with sysObjectID
detect=startswith('.1.3.6.1.2.1.1.2.0', '.1.3.6.1.4.1.24734.'),