A little research reveals some reassurance. Although botulism is a very very nasty toxin that attacks the nervous system and causes paralysis it is also extremely rare. Food is contaminated before preservation, and the bacteria germinate and reproduce in anaerobic conditions, producing toxin, which is then ingested. Foil-wrapped baked potatoes, sautéed onions and fermented fish are culprits, plus, let’s be careful here, garlic in oil. I have occasionally seen that garlic in oil ferments, but I didn’t know about the botulism bit.
Food with a high acidity is generally safe because the bacteria cannot live in an environment with a pH lower than 4.6 – lemons, pears and pickles generally have a pH of 2-4 (pears eh?). So we are fairly safe with the lemons it would seem, plus all that salt.
They (preserved lemons) do throw off a sort of sediment, and the lemon pulp particulates in the lemon juice also sink to the bottom of the jar, leaving clear lemon juice and a sort of lemon mud around the base. It does no harm. You can add more lemons and salt, and you need to top up the jar with lemon juice, but otherwise, if you keep the jar in cool, or chilled conditions, there should be no need to worry. As far as I can see they keep for at least six months or for up to a year, depending on who you trust.
It’s the garlic in the olives that worries me, but I think if I add some vinegar that should change the pH… Ah ha! I know! I will add some chopped preserved lemons to the olives - what a very good idea!