Big Beer Carbonation Time - bottle condition or force carb

I recently brewed a big beer (9.5% robust porter) for a friend's birthday gift. I started with bottle conditioning using corn sugar. After 4 weeks, the beer is still fairly flat. I understand that it can take a while to carb up in a bottle being a fairly big beer. How long does such a beer typically take to properly carb up? I'm thinking about abandoning the bottle conditioning, transfer all the beer to a keg and force carb, then beer gun it back to the bottles for transport as a gift. I don't want him to wait too long for the gift.

If I force carb the beer now, do I run the risk of over carbing since I already introduced corn sugar for the remaining yeast to do there thing? Maybe I should shoot on the low side of volumes of CO2 for force carbing to account for some additional yeast carb action over time??

I wouldn't risk oxidizing it by transferring back to a keg, then back to bottles. What temperature are you conditioning at?

Conditioning at 70F. My concern is that I want to give this as a present sooner than later. If I give him the bottles now, or in a month's timeframe, he would still have to wait for a considerable amount of time for full carbonation. I assume that it will carbonate in some amount of time...

If I were to rack to a keg, I would need to carefully pour out the bottles to minimize O2 intake, then purge for a bit with CO2. Has anyone done this before with success?