How can I keep a slow roasted pork belly roll warm while roasting potatoes?

It's normal to rest meat before carving, sometimes for quite a long time.

The oven will already be hot after cooking the meat, even if not quite as hot as you'd use for potatoes. so I suggest you parboil the potatoes (often recommended anyway) and put the roasting tin and fat into the oven to preheat when the pork is nearly done. Then take the pork out of the oven, put the potatoes in and turn it up. The pork should rest under cover on the container in which it was cooked. A loose (tented) foil lid with a tea towel or two over the top is good insulation. If you want to speed things up further, you could turn up the oven a few minutes before taking the pork out. The effect on the pork will be minor (in most ovens) but the potatoes will cook quicker and may be more crispy. I doubt you'll get very crunchy crackling at that temperature anyway.

170C isn't far off a good temperature for crispy roast potatoes. If they're a floury variety, and you parboil them, then when as dry as possible, and still warm, gently shake them to rough them up a bit in a bowl with the hot fat you're going to roast them with, they should take about 40 minutes at about 180 (turning once or twice,) to be golden and crunchy on the outside, and fluffy in the middle. Don't overcrowd them: steam is the enemy while roasting.

Crackling is helped by scoring. blanching the skin with boiling water, and drying as much as possible before salting generously.

(The pork crackling may require a higher temperature to finish, too. I would have no qualms about stripping it and putting it back in the oven while the meat rests, if necessary, if it doesn't wreck your carve.)

That aside, you should be able to overlap your cooking times. Put the potatoes in about 25 minutes before the end of the meat's cooking time, and then when you take the meat out, give the potatoes a turn, and turn the oven up to about 185C. This is assuming about 15 minutes resting time for the meat. More won't hurt it. The potatoes, on the other hand, don't want to hang around for too long.

I rest meat in a just-warm place; its surprising how little heat it takes to keep meat cooking and going too far. With pork belly that's not too much of a concern. If it's gone far enough, the crackling should improve while resting. Wrap the meat loosely in foil, letting it vent well at the top. You don't want the meat to steam. A bit of extra length in the foil end to end will let you make a spout from it, to pour excess juices from, maybe more than once. I usually make, or at least start, a gravy in the tin the meat roasted in - in my case, that's where the juices go.