Learn English – “It would be better if you drink/drank all the water”

In formal speech and writing, counterfactual clauses beginning with as if and as though take imperfect subjunctive, which means the were form in the case of to be, the only verb in English specifically marked for that tense. This the same tense you use with wish. For example,

• It looked as if it were already done.
• I wish it were done already.
• She dressed herself up as though she were a little princess.
• She wishes she were a little princess.
• He orders me about as if I were his wife. (but I’m not)
• He wishes I were his wife, but I’m not.

For other verbs, you just use the pluperfect there by using had learned, as you have done.

• He talks about Rome as though he had been there himself. (but he hasn’t)

There is some distinction to me made between whether the hypothetical is in the past or the present, leading to a simple past versus pluperfect=past perfect distinction:

• He acts as though he hadn’t eaten a decent meal for a month. (subjunctive about the past, so pluperfect/past perfect)

• He acts as though he ate a decent meal right before today’s race. (subjunctive about the present, so simple past)

When you’re talking something that the speaker supposes to be true, there’s no marked subjunctive in the present. It works like like then.

• He looks as if he knows the answer.
• He looks like he knows the answer.

As those examples show, although originally as if/though could only introduce counterfactuals, it no longer inevitably does so. Also, in informal speech, some of these niceties are no longer always observed. The alt.usage.english Subjunctive FAQ has more about all this.

According to English Grammar Online 4U, would can be used in a future tense as a Conditional I Progressive tense.

The conditional I progressive puts emphasis on the course of an action that might take place.

The phrasing might blurs the line, as in your example, September 5, 2015 almost certainly will take place and it is a documented fact that Freddie Mercury was born on that day in 1946. Let us focus on the conditional part of the tense instead. In a broader definition, would can be used in a tense that EnglishPage.com calls Past/Present/Future Unreal Conditional + Continuous.

FORM
If-clause: [were + present participle] + Result: [would be + present participle]

USE
Future Unreal Conditional + Continuous can be used like the Future Continuous in imaginary situations to emphasize interruptions or parallel actions in the future.

NOTICE The future form looks the same as the present form. The future is indicated with words such as "tomorrow," "next week" or "in a couple of days."

Breaking this down further, the Future (Real) Continuous tense is what you correctly identify as the will be usage, if he were alive. Unreal modifies this tense to work in the case where Mr. Fahrenheit is deceased (which is the current reality). The two key components to identifying this as the Future Unreal tense are:

1. Using a future date. You chose to name the exact date of September 5th, 2015, thus to identify it as a future date we need the additional context of the current date. If you instead started your example with Next year..., this would match the examples from EnglishPage.com exactly.
2. The implicit conditional. The phrasing you're mentioning is common enough that a native english speaker will understand that you're referring to a deceased person. Therefore, your sentence ends with an implied ... if he were still alive.

Putting this all together, the correct phrasing for identifying a future birthday for a deceased person is

September 5, 2015 would be Freddie Mercury's 69th birthday.

The form is reversed from the examples above, but it is a valid construction.

Subject (future date) + would be + present participle (birthday) + if clause (implied: if he were still alive).

As Tim points out, though, tenses are loosely enforced in English and many native speakers would correctly parse would have been correctly.