Update from this post. After some back and forth with the CS team they decided to escalate the issue.
To people who might not know, Welt's E6 effects do not apply to the damage dealt by talent, traces, and E1 when using skill or ultimate. This makes it a pretty underwhelming E6 since most of Welt's damage comes not from the skill itself, but rather from all his extra instances of damage.
This is different than for example, when using his sig LC “In the Name of the World”, as the effect provided by this LC does indeed increase the ATK% not only for his skill, but also for his other instances of damage such as the talent and traces.
You can see the difference in pic 2 (a simple quick test I just did) that the talent damage is higher when using skill than when using ultimate. When using another cone, the talent's damage remains constant between skill and ultimate (I did this with over 100% EHR so the conversion wouldn't skew the results from skill).
Also when using skill (with his sig LC), all the damage is increased by the 18% EHR from the cone by the EHR% to ATK% conversion (if you're under 80% EHR that is) from his trace “Punishment” during the attack (not taken into account in pic 2 if you're wondering).
Both his E6 and LC have ambiguous wording, E6 is “for the DMG dealt” and LC is “for this attack”, which might be interpreted as all the damage outputted by Welt when using skill and not just limited to the skill damage, and they work in different ways.
Hopefully they either change how it works or at least change the wording, so people don't build their E6 Welt with 70% crit chance.