There are many ways to cut it, depending on what you are trying to achieve and what you have available.
I usually try to use a sharp knife or chisel whenever possible. It doesn't stay sharp and needs regular attention, but it's the method that tends to cause the least airborne fibres, as far as I can tell. I use a shopvac with a fine filter and it still makes me nervous.
A Carbide bit in a router does a superb job, but you really need to be controlling the dust well. Outdoors, a good vacuum with dust collector on the router and a good particulate mask is what I have used, but I should make clear that I'm in my 60s and don't expect to still be around in 40 years. If you're in your 20s and/or intend/expect to make 100 in reasonably good health, this may not be the method for you.
I've used both a handsaw and a jigsaw with reasonable success. I feel these are probably both less unhealthy than the router, but probably produce much more airborne fibre than the knife.chisel. Again, outdoors and using a good vacuum with fine filtration seems sensible
Coating is dependent on what you are trying to do. I've used Sodium Silicate (waterglass) for HT "forges", that'll never get above 1100 degC, 2000 degF. Mostly, a suspension of Zirconium Silicate (Zircopax) in commercial rigidizer does well for "forging forges" IME, but for welding temperatures a layer of hard castable is probably best.
I've used a utility knife, aka box-cutter.
I suppose you could just coat it with some ITC-100 or other ceramic coating. If the Insboard was on the floor, or if you think it'll get abused by putting steel in and out, I'd use some refractory for extra protection. But if only on the walls/ceiling, I'd just use ITC-100
IMO, the only (and unfortunately major) downside to using Insboard is there's no thermal mass to store heat.
I pour a floor with Mizzou. The board is just for the end caps. The one that didn’t have to be cut off got board and Mizzou. Other end gets satanite. Upper part of the forge for a double coat of Satanite. Everything will get the ITC 100 before I tack weld the end cap back on. My forge is an old one, so it has a 10 inch body instead of the 12 in on the new ones. Also no pop off end cap.