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:
- 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.
- 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.
I would say your grammar book is wrong in saying possibility a (would wear glasses) is wrong and only possibility b (used to wear glasses) is correct. This gives a totally wrong view. Both are correct, but "used to wear glasses" is much clearer. The reader understands at once that you talk about a past habit. If you use "would" in a single sentence for past habits the reader may get on a wrong track as "would" has a lot of uses.
Mostly "would" for past habits is used in passages where the reader immediately sees that an author talks about past habits as in
After dinner my grandfather would always go out into the garden, sit on the bench under the apple tree and smoke a pipe.
Very often "would" for past habits is used in combination with "always".
See Oald, would no. 12: http://www.oxfordlearnersdictionaries.com/definition/english/would?q=would
Best Answer
Conditional Sentences:
Zero Conditional (facts - both parts in present tense): If it rains, water falls from the sky
First Conditional (real possibilities - if+present, will+infinitive): It it rains, I will use my umbrella
Second Conditional (here we talk about the unlikely - if+past simple, would+infinitive): If it were to rain, I would have to buy an umbrella.
Third Conditional (same as 2nd but in the past - if+past perfect, would+perfect infinitive): If it had rained, I would have had to buy an umbrella
Mixed Conditional (mix of time from the above - tenses can be mixed) If it had not rained, I wouldn't be wet
(all the above can also have the progressive aspect)
Conditional mood is the would+verb part in second and third conditional.
Mood and aspect can be combined:
simple aspect: would do (simple conditional)
progressive aspect: would be doing (progressive conditional)
perfect aspect: would have done (perfect conditional)
perfect progressive aspect: would have been doing (perfect progressive conditional)