Priming a GF sorghum beer, and bottling from primary

So I've got my first gluten free sorghum beer going in primary (an attempt at a possibly too-bitter IPA).

I'm still pretty new (last brewed from kits about 10 years back), so I do not yet own a carboy, and do not plan to do secondary on this batch. I also do not own a bottling bucket, though I may consider one for the future. I do own a 12-litre stock pot which I used as a wort kettle.

The LHBS recommended I prime using extract, and said I should use about 2/3 cup for a 6 gallon/23 litre batch. I ended up making a 5 gallon batch, but I did put aside 2/3 cup of sorghum syrup to prime with. However, I've recently been reading that sorghum tends not to give a good head (something I've seen with commercial sorghum beers). I have noticed that a local supermarket stocks sugar in convenient pill-shaped blocks in just the right quantity for priming one bottle (though I need to check if that's for pint bottles -- I have the 330ml ones).

So I have a few questions:

1. Should I prime with the sorghum syrup, or just use sugar?

• Given that I intend to bottle from primary, will it be easier/better to rack to my 12 litre kettle (in two batches) and mix in the primer there, or to stick the appropriate amount of sugar in each bottle individually and syphon directly?

• If I use extract, how do I work out the right quantity (so I end up with good head, but no bottle bombs)?

Thanks!