"Perhaps it is already so."
Translation:Magari è già così.
Forse expresses simple doubt, magari underlines the contrast to what's being said before; perhaps the speaker has been making plans and now he wonders if they're unneeded, as "it is already so". In other contexts (I don't think that's the case here) magari can be used for something one wishes (magari fosse così - I'd be glad if it were so).
I would stick with forse because magari has other implications, depending on the verb forms and even the region of Italy.
Già is an adverb which normally follows a verb when modifying it. Exceptions are unusual. If there is more than one verb, it follows the first, e.g. "it could already have been so" = ha già potuto essere così.
I'm not sure when you use an abnormal word order but I think I've seen (a) for stress and (b) in an idiom. Hard to use for we foreigners, especially English speakers.
No, you can't use subjunctive here, even though the sentence expresses doubt: it's just not correct. As a rule, you can't use subjunctive after "secondo me" (in my opinion), forse (perhaps), probabilmente (probably), and a few others, even though they express doubt. In the case of "magari" using it with subjunctive changes the meaning from doubt to wish.
To add another perspective, I've just read a post in http://italian.stackexchange.com/questions/5392/differenze-di-uso-tra-forse-e-magari that suggests magari is said with hope, confidence or probability (its root is the Greek for felice/happy) and forse is said with doubt, uncertainty or possibility. The hope element is what makes magari also the common idiom for "I wish" and "if only!"
It also says these basic elements vary regionally, which makes them a minefield for foreigners, e.g. you risk communicating things like "hopefully the plane has crashed".