Is it grammatically correct to use 's to pluralise an acronym?
More than one FAQ: Is that FAQs or FAQ’s ?
Is there a possessive form, such as "The FAQ’s content is. " ?
FWIW, I prefer to pluralize acronyms just like any other word: FAQs, ASPCAs, TANSTAAFLs. The possessive form should use apostrophes, as in your example.
With a name like Q.E.D. you must be an expert in this topic 
If the acronym is in caps, I use “s” for plural: FAQs. But if I was using faq, I would pluralize it as faq’s, faute de mieux. Compare MIPS (million instructions per second) agains MIPs (plural of MIP, not that I know any such acronym. It is not really a question of grammar but of spelling.
Older style guides said to use 's to make plurals.
More recent onces lean toward just adding the s – no apostrophe.
It’s a matter of style and it depends upon what style manual you use. The federal government uses an apostrophe in such plurals, but the Chicago Manual does not. FWIW, I had a long argument with James J. Kilpatrick regarding this subject years ago. He insisted that the apostrophe was incorrect in numbers but correct in letters: 9’s, for example, is not correct; however, he did allow for certain exceptions, such as “watch your p’s and q’s,” his theory being whenever the lack of an apostrophe can lead to ambiguity, it should be inserted.
As a joke, I sent him a summary judgment based upon my petition, signed by an administrative law judge, in my favor, based on numerous contentions, not the least of which was discrimination without any rational basis.
As you may know, he describes himself as a Court sometimes, and in a subsequent column he said that he would allow no further appeals.