As for trying to stay in a zone for racing, whether mountain, road, or cross is a huge mistake. You need to push as hard as it takes for as long as it takes to stay ahead/with the group/situation as it takes.
Not really, the answer is, “it depends”. Knowing and riding within zones can be invaluable. Going into or above Z6 is danger territory - known as “going in the red” as it will deplete you very quickly. At the opposite end of the scale most fit riders would be able to sit at Z2 for many hours. Depending on the type of race your doing, in general you’ll be training yourself to ride the fastest speed in given zone.
So, for a short XC race of 90 mins, most riders will be in zone 6 and 5 all of the time therefore you need to train a) to be able to manage that and b) the nature of the race which will be short sharp accelerations out of corners / overtakes.
For a marathon race of say 4hr - this is me - you me sitting at the top of Z3 or bottom of Z4 for that period BUT there will be times you’ll be in Z5 trying to avoid Z6 such as long climbs. Here you need to train Z2/3 to enable you to sit in Z3 at the highest speed possible and z5 /VO2max to support those shorter yet higher efforts.
For longer distances - 6-24 hours it’s going backward further - you ride a 24hr in bottom Z2 for example.
Very much in the latter two you’ll have time to glance at power / hr to manage your efforts.
Yet as above, MTB can be vary variable due to the nature of the terrain so you will get spikes and troughs in power and HR. XCO is so rapid you shouldn’t really even have time to look at HR.