What Maylar and Rick say is correct, it is an easier process to ferment fully then back sweeten with enough sugar, juice, AJC or whatever to take the cider back up to the sweetness level that you want, plus a bit to be consumed by the carbonation process.
I try to capture the "sweet point" on the way down in order to keep as much "appleyness" because I understand that some of the apple nuances get "blown off" as the SG approaches 1.000. I must say that I haven't really noticed any difference between the two processes in this regard, so the flavour "blow off" bit might just be a bit of folklore.
Once the cider is bottled, I use a test bottle fitted with a pressure gauge to monitor carbonation buildup in order to pick the right point to pasteurise. This is just a sophisticated alternative to Rick's coke bottle approach which I have also used. The only downside that I can see with the "coke bottle" method is that you do need to monitor the buildup of firmness (it takes a while). You need to know when it first reaches normal soda/coke firmness, because with excess pressure (i.e. let the carbonation go on too long to something like 4 volumes which would be around 200 psi at pasteurisation temperature) the coke bottle will still feel firm. The gauge approach lets you know if the pressure is right for the carbonation that you want, but it is probably only worth the trouble if you plan to do a lot of carbonating. Maylar has a post on making a pressure gauge test bottle... and as Rick says elsewhere, you can drink the test bottle. (I do for "quality control"!)
I don't know what bottles you are using, but pasteurising at 70C with 2.5 volumes of CO2 will generate around 125psi pressure before it returns to something like 30 -45 psi when the bottles cool down. Normal 12 oz beer bottles are batch tested at between 200 and 250psi. Within any batch of bottles there will be some that are under-spec by up to 20% so any "normal" bottles should comfortably handle 125psi for the short pasteurisation period without bottle bombs.
As with anything of this nature it is wise to wear protection equipment such as gloves, goggles, etc because of the potential bottle pressure when pasteurising.