You're right to think about air flow; ventilation to the underside of timber floors is essential. You'll probably have air bricks - which you can see on the outside walls at low level - designed to allow air flow beneath the floor. You just have to ensure the air flow is maintained - which really means insulating between the joists only. You need to avoid bridging any air inlets or means of cross ventilation. I would choose a contractor with the right experience.

as all the above, it is a good idea as long as you don't block airflow underneath. However it will not be an easy job, either meaning lifting floor boards throughout or crawling under (depending on the depth of the void). If you pay a builder it will not be cheap and will be very disruptive. There are better options which would be far more cost effective and easier to do.

As was common during the Victorian era, when suspended timber floors were prevalent, airflow should be maintained via grilles ideally in opposite external walls across the crawl space and not through the floorboards as that would create an unwanted draught. It would therefore depend on the amount of insulation and the depth of it and its relation to the location of the grilles in this 1930s context to make a judgement as to whether this would prevent air flowing between the grilles/outlets in the external walls.

There is absolutely no reason why you should not fit kingspan insulation between the floor joists. you just need to make sure you did not obstruct the air flow through the air bricks and if there aren't any errors get some fixed in position to allow for a good airflow.

Insulation manufacturers Kingspan have produced a You Tube video explaining how to insulate between the joists of a suspended timber floor. Search Kingspan Insulation Suspended Timber Floor on You Tube. It may be necessary to approach a building surveyor for advice on the amount of insulation to install or contact Kingspan Technical department for advice.