Hi Darren - headspace is measured from the top edge (the rim) of the jar. Exploding jars can happen but it's rare if directions are followed (checking jars first, using only approved jars, adding hot liquids to HOT jars, remove rings, etc.)

If you are going to be doing some serious canning then buy yourself a copy of the Ball Blue Book of Canning and Preserving (soft cover sold wherever canning supplies are sold) as it is the bible of canning recipes and detailed safety directions.

The use of rubber gaskets is OUT and has been for some time ;), not everything can be processed in a BWB, and "normal canning procedures" is determined by the food being processed so varies with each recipe. In other words, safety rules have changed and old habits have to change with them. You don't want to count on knocking on wood to make safe products. ;)

Liquid vs. powdered pectin is just preference. Some swear by each camp.

Never made any oils so I'll let others cover that point.

Dave

PS: welcome to the forum and the wonderful world of canning!! :^)

Here is a link that might be useful: National Center for Home Preserving

You cannot safely can oils. They need to be made fresh and stored in the fridge for no longer than a few days. I know this article below says 10 days, but we are now recommending a shorter time than that. That, or freeze them.
Oil cuts off the air supply. That is how botulism grows, in the absence of air. Plus, you give it a nice food source in the things you add to the oil.

Here is a link that might be useful: Safety in making flavored oils.