Scorching Ray Witch - Questions on Build and Mechanics

Hello

I am a returning player and have started a Witch in the Delirium League HC. Not sure if I am posting in the right forum, but here we go:

I wanted to try Scorching Ray as my main damage spell and now am kind of looking for clues to develop the character. Also a bunch of questions on certain mechanics have come up which despite research I could not find an answer to. And what I have found was difficult to grasp with the little knowledge I have.

But first things first. I have developed the character (lvl 38) this far in the passive skill tree:

https://pastebin.com/fzXbERa3

(I have not taken any keystones yet and I think I still can develop in any direction on the skill tree.)

The general idea so far has been to improve

Main Attack Skill: Scorching Ray
- Improving Cast Speed
- Improving fire/elemental/spell dmg

Defense
- Going for Energy Shield as main defense
- Using a shield to boost defense (so no two-handed-weapons)
- Leeching energy shield with secondary attack?

Secondary (Attack) :Skill ???
- Herald of Purity
- Armageddon Brand
- Soulrend
- Vaal Righteous Fire (found one, will be left for later consideration)

Utility skills: Auras/Curses turned to Auras
- Herald of Purity
- Enfeeble
- Elemental Weakness

Movement Skill
- Frostblink

Additional Defense Skill
-CWDT/Immortal Call


I am currently killing everything pretty swiftly, which is to be expected at this low level. I feel I am lacking defenses and I am unsure how to develop the character so it will stay alive in HC and still have decent clear speed.

Now to my questions regarding the skills:

Herald of Purity
Pretty nice skill but I am not sure if it will stay viable later on. It kind of does a decent job in adding to dmg and keeping mobs busy but I am not sure if it is worth developing and if yes, how? How can I keep it viable? There seem not to be many synergies with the rest of the build.

Armageddon Brand and Soulrend
I started leveling those gems as I am looking for ways to enable the build to leech energy shield. As Scorching Ray does not HIT I cant leech anything (bummer) The idea is to connect either with Cast while Channeling. Not sure which to use. Armageddon seems to scale nicely with the fire dmg and Soulrend does exactly what I want: leeching Energy Shield. Though it does not energizes as nicely. Is Cast while Channeling an option to go with, or shuld I rather invest into more support gems that directly improve the SR dmg?

Enfeeble & Elemental Weakness & ???
With previous characters I used multiple curses as utility to improve survival and boost dmg. But running these curses and the Herald is not possible, so I think I have to ditch one or the other sooner or later. Any suggestions if the Curse/Scorching Ray combo is enough to carry me to the endgame?

Frostblink
I like this movement skill. The elemental dmg is nice, it freezes/slows enemies. Only I am not sure what kind of support gems would be good to get the best out of it. Neither faster casting nor faster attacks seems to reduce the cooldown.

Question on Mechanics:

What is the difference between fire dmg, spell dmg and elemental dmg. They all seem the same to me in the end. What does scale best with SR? How do these interact with the skill if improved?

I noticed something strange. I have two one handed weapons. A wand and a scepter.

The wand

The scepter

If I put on the wand the DPS meter goes up to about 700dps.
If I put on the sceptre the DPS meter sinks to about 350 dps

Strangely I have the feeling that monsters die actually faster with the scepter equipped. Its hard to tell as they usually die really fast but there it is.

As you can see the added bonus on the scepter seem to be better fitting for SR in general but the DPS calculation says otherwise.


SUMMARY:

1) How should/could I develop my defenses
2) How doe the mechanics with SR work?
3) Any advice regarding the skills mentions above?

Many, many thanks in advance for your advice and help!

S.