Question: Reginald Hornfather's Battle

You're correct. We can also eliminate B due to the missing "so."

However, please note that the "just as X, so Y" construction is used for comparisons/analogies.

There are times when the "Just as X..." construction doesn't require the "..so Y" part. For example, we can use "just as X" alone if we want to show that an event happened at the same time as another event.

For example, we can write "Just as we walked in the door, Joe started to vacuum."

The "just as X, so Y" construction is used for comparisons/analogies.

In your example "Just as we walked in the door, Joe started to vacuum," the "just as" construction is NOT used to compare two things. Instead, it is used to indicate WHEN something occurred. That is, AT THE MOMENT when we walked in the door, the vacuuming began.

To answer your question, the past perfect tense (had started) is used when the action occurred in the past BEFORE some other completed action in the past. In your example, the vacuuming does not occur in the past BEFORE some other completed action in the past. So, we wouldn't use that tense here.

More here on past perfect tense: https://www.gmatprepnow.com/module/gmat-sentence-correction/video/1178

In many cases, "just as" means "at that very moment."

For example, we might write "Just as I walked through the door, the telephone rang." This means the phone rang at the very moment I walked through the door.

There are two problems with "just as" in the sentence.

First, Reginald and Bartholomew were killed in different battles, which may or may not have occurred at the same time.

Second, and more importantly, the word "eventually" doesn't match with the instantaneous meaning of "just as."

If we write, "Just as I walked through the door, the telephone eventually rang," then at the very moment I walked through through the door, the phone EVENTUALLY rang.

I don't think we can make the interpretation you made in sentence 2 (B.S. is eventually killed by his own men during war, who is like a Reginald Hornfather)

Just stick with the simpler rule that says: In a comparison, "like" must be followed by a noun, pronoun, gerund or noun phrase.

Cheers,
Brent

If we want to use JUST AS to make a comparison (regarding the manner in which the two men were killed), then we need SO, the 2nd part of the JUST AS...SO correlative.

i.e., JUST AS Reginald H was killed during the war of 1844, SO Bart S was eventually killed by his own men.

Cheers,
Brent

Oops. Sorry about that. I've changed the screenshot.
Thanks for the heads up!

Yes, I think that's a good reason to eliminate B.