I'd guess you'll not be needing to leave it on the fruit for much longer, at which point you can fill up a carboy and not leave headspace. Should be fine. Four days on the fruit in secondary could already be enough. How does the fruit look? When the color goes from the fruit, you can probably move it. I'd not leave it there for more than a week in any case.
I'd say don't touch the acidity now. Let the mead age a little before deciding if you want more or less acidity. I'd say the same thing about backsweetening, wait a little longer to decide. Whipping is probably not the best idea, but if you can add the honey without adding air that should do it. If you're not in a hurry, you can just pour the honey in and let it dissolve on its own. Pre-mixing with a little water or mead helps it go in faster.
I would not use a fining agent just yet, it's too soon. Fining agents are for small particles that are stubbornly not dropping out, not large particles that will fall out on their own. Patience, grasshopper.