But, it all works out ok in the end. If the jam is too liquid, I'll just change the name of it and call it "sauce" and serve it up on cake or ice cream!
Yes, a good cook can almost always remarket her "lesser successes" as something else or tweak them to improve the flavour..... not that I am by any means a good cook, but have had plenty of opportunity to rebrand my failures

As regards using apple to help set jam, I think it may be important to have the right sort of apple.... a good cooker.... (I inherited the apple tree along with the cook book!) and I usually just quarter it and throw it in the pan of boiling fruit and sugar and lemon juice and the squeezed lemon halves and then fish what's left out later. Or you can be less cavalier and put them in a muslin bag. It may be more effective to peel it and just use the peels as @NikAndHerChicks suggested. I might have to try that next time.