It may be important to note the temperature line for some of this. At 220 degrees F (104 C) the cider should jell without issue, so up to 219 theoretically you can maintain syrup/molasses. If the agitation is lacking a lumpy syrup may result (been there).
Regarding moldy syrup, the standard reduction is 7 to 1. It is more likely that something like maple syrup will mold more readily if not canned, due to the low acid content, whereas cider syrup may be off the charts for acid. This last point bears mentioning for those who are accustomed to a the subtleties of maple or other lite syrups. Cider syrup, and jelly from a boiled syrup can be pretty intense. We have found that truly boiling the cider results in a stronger brew (more molasses like). Generally, you are going to increase tannin, acid and other non volatile chemicals along with the sugars. Awesome to this boy, but can take some getting use to.
Since the outcome is similar to the reduction created through freezing, taking the temp in the opposite direction can help concentrate things. We can freeze the cider, like we would in ice cider production or apple jack making. Around 25 degrees F should allow for the pure water to freeze and leave a sugary mass as liquid. This can then be further reduced with heat.
I hope most folks with the set up to do so, to use passive or renewable means to reduce. Wood fired evaporators, residual heat from wood stoves, low temperature evaporation, and natural freezing are good starts. Even if the bulk of the reduction is done with these methods (for syrup or other fruit jellies) and finished up on the stove, we can help reduce the use of fossil fuels for our processes.
Walden Heights Nursery & OrchardZone 3 in Vermont