grammar - 「こわいだから。」 versus 「こわい、だから[...]」

In an answer on another question, @TsuyoshiIto wrote:

[U]nlike commas in English, 読点 in Japanese is rarely (if ever) grammatically required. Authors are free to use 読点 wherever they feel that it makes sense to make a pause when pronounced.

And in a comment on yet another question, @dainichi wrote:

I think [こわいだから] might exist in some dialects, but in standard Japanese, it is ungrammatical regardless of formality. Unless it's こわい、だから[...] which is something else. _{(emphasis added)}

What is this "something else?" According to answers on two other questions (this one and this one), だから is simply だ + から. Therefore, I would expect *こわいだから to be ungrammatical because *こわいだ is ungrammatical. In fact, I would expect だから to only follow things that だ can follow.

However, I'm having trouble reconciling what @dainichi and @TsuyoshiIto wrote. If *こわいだから is ungrammatical, and 読点 aren't required, how can こわい、だから be grammatical?

Perhaps my confusion stems from the difference between these forms:

1. *こわいだから。


2. こわい、だから[・・・]


3. こわい。だから[・・・]

Can だから be explained as だ + から in all three cases? If so, does that mean だ can begin a sentence or clause, or is だから somehow special? What is the "something else" referred to by @dainichi?

In short: how can I make sense of this?

Answer

I regret not writing "こわい。だから[...]" with a 句点 in my original comment.

The precise way to express it would have been: if こわい and だから are parts of separate clauses, it can be grammatical, otherwise not.

In speech, you would usually express こわい and だから belonging to different clauses by inserting a pause. With no pause between them (i.e. without breaking the mora rythm), it sounds wrong.

In written language, what is "correct" is mostly a matter of style and conventions. Maybe I originally kind of "broke the rules" by making it a 読点 instead of a 句点, but I would argue that it's not uncommon to join multiple main clauses with 読点 instead of 句点 if they're closely related. But I think most readers would find こわいだから without a 句点, a 読点 or a space very hard to parse, and would assume you had made a grammatical error.

Maybe one rule of thumb could be: If originally it could have been a 句点, at least have a 読点 or some other kind of space (the latter might be more common in poetry).