I take that to mean I could siphon it out back into my primary, clean/sanitize the carboy, then put it back in...then top off? If I go that route, how do I minimize oxidation? Just cover/airlock my primary bucket and do things as quickly as possible?
Oxidation is not a rapid process. It is a factor of wine volume vs. air exposure (head space) vs. time. A small volume of wine with a large headspace oxidizes faster than a large volume with a small headspace. Your wine will not oxidize in the length of time that is required to rack the wine into a primary, clean the carboy, and rack back.
This is not to say that air exposure should be taken lightly -- just that it's not the boogey man. Do things efficiently but without panic.
It's also important to add K-meta at rackings, as K-meta neutralizes harmful things (such as O2) by binding to them, which uses up free SO2. That's why we have to add more K-meta.
Sorry if this is an overly obvious question, but what would you consider a comparable wine to top it off with?
Nope, it's a very good question. A commercial apple wine would be a good choice, along with anything lightly flavored such as a Pinot Grigio. I would not use Chardonnay as most are strongly flavored, nor would I use a New Zealand Sauvignon Blanc due to the typically grassy flavor.
Another option is to stabilize the wine (sorbate + K-meta) to prevent a renewed fermentation, and top with apple juice and/or frozen apple concentrate. This will improve flavor and aroma, but it will reduce the ABV. IME fruit wines need at least a bit of backsweetening anyway.
While it doesn't help you today, for future reference take batch size into consideration. Always plan that you'll have more wine than your primary container, so you have top-up when you need it. If making something new, post a question about batch size.
