Yep, if you want to avoid a pectin haze in your cider always use pectinase ... and not just when you are heating it. And even though much commercial "apple juice" has been processed to remove much of the pectin ... treat it anyway ... it won't hurt and it will help insure a better result.
As far as the effect of high heat on the pectin - it's true that it will help set it making it much more difficult to avoid or clear a haze. High heat also kills some of the natural pectinesterase enzymes that would normally assist in breaking down the pectin. But you do not have to have heat to get a pectin haze ... as well there's a number of other ways ciders can get a haze or discoloration.
In any regard, for a clearer cider, using pectinase before fermentation is the way to do go ...once alcohol production starts, the pectinase begins to denature.
Different pectinase enzymes have different heat tolerances and pH preferences ... however there are 3 main pectinases that are usually combined in the pectic enzymes you buy ... but unfortunately, the most common brands used by home vintners (ld carlson etc) ain’t sayin which ones they included or what specifics to use them at.
So ... some general guidelines ... assuming that the primary pectinase you’ll get at the store is mostly polygalacturonase ...
The temp range is 59*F to 86*F ... and the pH is at least pH 3.5.
So, a must temp at a minimum in the mid 60’s for about 4 hours should be fine for grapes ... and for cider (or for that matter, any apples, crabapples, or cranberries) I’d say upping the standard dose to about double what is recommended and letting it go 6to 8 hours or even overnight.
If possible do it without any sulfites added ... but if you have any questions about microbes (such as you pressed the apples ... are using fresh pressed cider ... are fermenting on the skins, etc ... rather than using store bought juice), adding the sulfiting is not a deal breaker. Better safe than sorry - add the k-meta ... wait a bit for it to do its thing ... then move on to the pectic enzyme. In a moderate dose it does not affect the enzyme all that much.
For pH, you’ll see commercial pectinases performing best at an isoelectric point well into the mid 4 to mid 5 range (pH4.5 to 5.5) ... these are often the pectinases used in the production of fruit juice etc ... for winemaking pectic enzyme a pH in the 3.4 to 3.8 range is fine, though its true that a little bit higher would probably be even a little bit better ... say in the low 4's ... but even at that, it is likely a bit higher infection risk.
Similarly there are recommendations for using those industrial pectinases at about 115* to 120*F ... again, without confirmation of that from a data sheet for the brand and product you are using, I'd stick with the temps I stated above.
