It is spoken English, and it seems that the speaker changed their mind about how they would express something:
If we start where we left off in the last video ...
probably intending to continue ... I will be able to explain about the stars.
But then changed their mind and decided to review the start of the last video (So they won't start where ...
This kind of "introductory if" is a pretty common idiom, particularly in academic English, when presenting things to an audience. (Actually, I believe it's a cross-linguistic phenomenon — I've encountered its equivalent in several other European languages too.) You'll often hear presenters say things like:
"Now, if we look more closely at ...
The Cambridge Dictionary provides this definition and 3 examples. The first example is precisely on point:
away adverb (CONTINUOUSLY)
continuously or repeatedly, or in a busy way:
I was still writing away when the exam finished.
Chris has been working away in the garden all day.
We were chatting away at the back and didn't hear what he said.
Harry looked great. He had been wearing his new suit.
To me those sentences imply that Harry had stopped wearing his new suit at the time he was seen to be looking great. The past perfect continuous puts the wearing in the past of another event that is in the speaker's past, and the only event referred to is the time he was seen. Compare
Harry looked great....
You don't need the "in" in "features in a wide spectrum." The way you're using it, "features" is a transitive verb, with "spectrum" as its object (like "the hotel features comfy beds"). (The construction "features in" works in reverse: "Comfy beds feature in this hotel's best rooms.")
The highlighted sentence describes something that happens habitually or repeatedly, and doesn't need to be in past tense.
I have my laundry done whenever it gets dirty.
If it is your sentence, it seems alright grammatically. It seems harder to read because of the structure, though. The same thought might be expressed by leaving out the reference to people.
A proper noun is a specifically-named person, place, thing, or idea. A common noun is any other person, place, thing, or idea, i.e. all nouns that are not proper are common.
Looking at the passage, I count five proper nouns (all of them multi-word phrases) and fourteen common nouns, a total of nineteen:
Both are okay
It depends on whether you want to emphasize the negation of the second item in the list
When to Use 'Nor' (Quick and Dirty Tips):
“Nor” doesn’t necessarily have to appear in a sentence with the word “neither.” “Nor” can start a sentence.
How to Use Nor (wikiHow):
Use "nor" with other negatives. Even though "nor" is almost ...