Avoiding poisonous quince juice

Short answer: No.

Hydrogen cyanide (HCN) boils at 26 C (79 F), which is quite low, so after one hour of boiling you would be left with very little HCN. You need about 200 ppm in most mammals for the LD50 (dose at which 50% of a group of animals die). You are very unlikely to reach this concentration in a kitchen with the window open or an extractor fan running. I've been unable to find values for the HCN content in quince, but there are closely related species like apple and pear that do have available values for the HCN containing compounds.

However, what is actually in quince seeds is a nitrile compound - this is a group of chemicals that contain a cyanide group; the most commonly studied and mentioned form of this is a chemical known as Amygdalin. These are altered by the stomach acids and enzymes and can release the HCN from the nitrile. How much is released is probably a bit dependent on the conditions under which it is treated. Boiling seems to be effective at extracting amygdalins and that they contain in apple and pear between 3.0 and 1.3 mg/gram of amygdalin respectively. This converts to 0.1772 mg HCN per gram of seed dry weight for apple (please check my maths - HCN M_r = 27.0253 g/mol; amygdalin M_r = 457.429 g/mol; there is 1 HCN per amygdalin). If you boiled 100 grams of the (dry) seed you would get ~12 milligrams of potential HCN. The NIOSH limits for HCN exposure are a time-weighted average of 10 mg/m³ over 8 hours, so you would need to absorb all of that amygdalin over about 8 hours to see any effect.

It seems that the seeds are incorporated into a bunch of remedies (see sections on edible and medicinal uses), so it is likely that there is actually much threat from the seed or products of the seeds unless consumed in significant amounts.