Scope of Answer and Safety Information (please read)
This answer assumes that "preserves" refer to fruit preserved with sugar. It does not apply to vegetables or meat, or to cases where large amounts of sugar are not added. If canning in those environment, please use more conservative approaches, or better yet from a sustainability perspective use more traditional means of preserving these foods for later, such as sauerkraut and salami.
Fruit is not considered to be a particular risk when canning and if you go back a thousand years or so fruit preserved in sugar was stored open. The acidity and sugar together combine to prevent harmful bacteria from growing, and white sugar is considered, at least by archaeologists, to be better at preserving fruit than honey is (see "Anglo-Saxon Food and Drink" by Ann Hagen).
Additionally there are a few fruit which have fallen out of favor which are traditionally preserved with citrus added, most notably the quince ("marmalade" comes from a word meaning "quince jelly"). If going outside of the common fruit used for such preserves, you may want to test pH (which may be a good idea anyway). If in doubt, add some extra lemon juice or other acidic fruit.
Additionally, even in the Middle Ages, such preserves were cooked, both before bottling and after bottling, so you can't omit the water bath.
For fruit preserves, a pH of 4 gives you extra room in case the pH rises due to any number of factors. Also note that fermented foods generally (that includes either lactic fermented foods like sauerkraut and alcoholic fermented foods like beer) are not botulism risks.
For more on safety, please see:
As always, defence in depth is important and canning is a part of that depth today when making fruit preserves.
The old traditional method of sealing was to use wax. There's no reason why a "float" of beeswax won't get you where you need to go along with jars which cannot themselves be sealed. While this often works it is not perfect and the seal can break. This being said there isn't any reason why this can't be used as one aspect of a seal. It may not be perfect, but it is simple to do, and if the product is used within a few months, it is likely sufficient.
I would prefer beeswax to paraffin not only for sustainability reasons regarding the material itself but also because beeswax is softer stickier, and less brittle and so I would expect that it would be less likely to come loose.
Another option, as you mention, is to re-use existing jam jars and lids. I have done this with herbal medicine concoctions. This usually works pretty well but some sorts of jars and lids I have learned to avoid not because they won't seal but because sometimes the seals end up locked and the jars impossible to open. Jars to avoid include baby food jars. I would assume that other similar jars should be avoided too. Note you can test a seal with these by virtue of the fact that the button pops down. If this doesn't happen, try a wax seal on these, and use first.