In American English usage, many writers and editors feel that a comma should precede and with three or more items in a series (Oxford comma). Usage differs in other countries. “Optional” means you do not need to use it unless you are submitting a formal document to someone who insists on using it.

A dependent clause does have a subject and predicate; however, it does not express a complete thought on its own (see examples of dependent clauses in this post). Your sentence is grammatically correct using the adjective “plentiful” instead of “plenty.” The sentence does not contain a dependent clause. “Because of the plentiful harvest” is an adverbial prepositional phrase. See Clauses and Phrases for more information.
The farmers are happy because of the plentiful harvest.

As written, the statement incorrectly uses a comma, which results in a fused run-on sentence.
The content includes two independent clauses that can be written in three different ways:
1) I like this class. It is very interesting.
2) I like this class; it is very interesting.
3) I like this class: It is very interesting.

The sentence begins with two dependent clauses that modify one independent clause. The second dependent clause has an understood omission.
When there was one patch that was super clear and (when) you could see the bottom, I then believed I was done washing the dish.
We would leave the comma out if “when” is omitted, but we would put the comma in if “when” is not omitted:
When there was one patch that was super clear and you could see the bottom, I then believed I was done washing the dish.
When there was one patch that was super clear, and when you could see the bottom, I then believe I was done washing the dis

When there was one patch that was super clear and you could see the bottom, I then believed I was done washing the dish.

Our Rule 5a of Subject-Verb Agreement says, “Sometimes the subject is separated from the verb by such words as along with, as well as, besides, not, etc. These words and phrases are not part of the subject. Ignore them and use a singular verb when the subject is singular.” Therefore, “is hosting” is correct.