"If it were up to me..."
That's the correct grammar for when you have a conditional argument. I know that a lot of Americans don't know this rule at all (especially musicians) but I suspect that it is a regional thing and not strictly US vs. Brit.
Note that if the conditional reflects something that really happened, then you use was as usual.
"If I was wrong by saying that..."
- Hey, how did you know he was the perp? He wasn't even on our suspects list!
- Remember when we asked him about our victim? He said, "If it was up to me", not "If it were"... which clearly suggested it was up to him.
As far as musicians go, you can find examples of people on both sides of the pond using or not using the subjunctive. Beyoncé sings "If I were a boy," while Thom Yorke sings "I wish I was special."
In English, what looks like a plural form ("were") in place of a singular ("was") actually comes from a Germanic singular form of the subjunctive. Both the German words "waren" (plural indicative form - "we/they were ...") and "wäre" (singular subjunctive form - "(if) I/he were") become "were" in English.
(And to confuse things, in English, the plural subjunctive is also "were" and is thus indistinguishable from the indicative. "We were kings..." v. "If we were kings...")
In both languages, the subjunctive is used in this case for a condition contrary to fact - I am not the King of Germany (nor am I a German pop singer from the 80s!).
For example:
"Que tengas un buen día,"
would translate to:
"Have a nice day."
So, in Spanish the subjunctive is not strictly used for conditional situations. It is not really that you consider that the other person may have a bad day. The closest way to say that in English is: "May you have a nice day."
Like a lot of commentators are observing, it is not that clear whether you need a subjunctive. In fact, if you look at Afrikaans, there are only four tenses: past, present, future and historical present. And Afrikaans is considered a full scientific languages: You can translate any scientific article into Afrikaans.
On the topic of languages from South Africa, you also have noun classes in all of the 9 official languages that are not derived from European languages. These classes are a type of concord that have nothing to do with tense, but rather with types of nouns. They are also not strictly necessary, but they make it easier to follow a conversation. One way in which they achieve this is that if you say something like "I have an axe and a shovel," and later that "I often use it in the garden," then you will be able to know which one of the two you are referring to since they are in different classes and the word "it" need to be in concord with the noun. If they are in the same class, then you don't have concord distinction when referring to the object. I think the development of this feature are more to distinguish objects and subjects in sentences, as they are more likely to be different types of objects (e.g.: a human and a thing) and hence more likely to be in different noun classes. An axe being in a different class than a shovel is more of an coincidence.
"The doctor insisted that he spend the night."
vs.
"The doctor insisted that he spent the night."
The first sentence is how I would say it.
The second sentence sound very British to me.
I would use the second sentence for both cases. If I needed to distinguish them, I'd probably add something like "last week" or something to clarify things.
No, it's subjunctive vs. indicative.
The British form is indicative and past tense, which is not correct grammar under either British or American rules, but is used in common speech.
Replace the verb with "to be", e.g. "the doctor insisted he be here." vs. "The doctor insisted he were here." to illustrate that the past subjunctive doesn't fit.
The past subjunctive is almost exclusively used for hypotheticals, e.g. "If I were rich...."
This isn't hypothetical. This is a mandative statement, so we use the infinitive form of the verb even if the action took place in the past.