opinion???

The van with the sliding door would be an advise from me. It's not a passenger door.

Not sure about the spare wheel under the bonnet. Would probably treat it in the same way I would a battery. Remember there are no what if's in an mot. i wouldn't class a spare loose in the boot well as insecure because its not going to go anywhere.

Tested a few camper vans. Would say your right in what your saying about 3 wheels locking. If not do a decel.

For tyres I divide the with by 8 then use the tape measure to finds the central 3/4. If in any doubt I advise that the edges are bald/heavily worn etc.

With tyres, I use a tape and measure from the centre out and knock of a third.

Gaz im well impressed m8 learnt something new today

So you are then measuring the central 2 thirds of the tyre not 3 quarters. Measure out from the centre and knock off a quarter instead of a third. I use the same method as Gaz personally to find an eightth, then set my tread depth gaugeto tha result, then set the green flange of the gauge on the edge of the tread with the measuring 'stalk' facing inwards, and the tip will be resting on the edge of the central three quarters.

Had a customer argue a tyre with me today. He'd been to a tyre centre and they told him his tyre was fine.
A 225/45r17 on the back of a BMW 1series MSport. Measured the width of the tread and it was exactly 200 mm, so easy to divide by 8, measured 25mm in with my depth gauge and stood it in the groove and it showed 1.48mm (I used a digital)

Customer said. "Tyre centre told me it was 1.62"

1.62!!!! FFS! Do they not want to sell any tyres?

After watching Clarkson lose a 1-MSport on the top gear track this week on decent tyres I can't believe anyone would want to drive one with less than 2mm of tread left

As for the camper weight? Test using the base vehicle's weight on the computer. Same as you would a Berlingo van fully racked out with shelves full of plumbing stuff.
Unless your rollers take a weight measurement of course.

Going back to your first subject, the cargo bay door, If it opens from inside so you can do a proper inspection, or if it doesn't open at all, but you think you can do a proper inspection with it closed, then it's a pass and advise.

Looking at each point in turn.

Sliding/opening doors of goods vehicles are not passenger's doors, they are loading doors. If during the pre-checks that any door does not open then this is a justified reason to refuse to test that vehicle. If a door cannot be opened, lets say a side loading door, then if that B post had a problem like a fracture within the seat belt mounting area(s), you are sure to not observe that defect. 

The spare wheel.

I suppose it depends on where the spare wheel is mounted when deciding if insecurity becomes a problem, if the NT decides that an insecure spare wheel is a cause for concern, then again the vehicle should be refused an mot test, however I would probably advise the NT to seek a second opinion from VOSA first as their interpretation of the meaning of insecure is such that free movement of a component is not necessarily a reason to prevent the test being carried out, nor a refusal to test being issued. 

Unknown Vehicle Weights

Where a vehicle is presented for a test of unknown weight, and is a motor caravan not being a living van, then the NT must decide before registering for the test if that vehicle falls into class 4 or 5, which depends on the seating capacity and has no bearing on the vehicle weight. If the size of the vehicle is obvious it can't fit on the approved equipment then there is no problem, however if it does the NT might consider thinking about the lifting capacity of the ramp and jacking beam(s), and if in doubt I would like to see a weigh bridge certificate to satisfy me, which can then be used for the presented brake test weight of the vehicle.

My experience has shown that with the new brake test procedure on our equipment I never see two wheels on the same axle lock out, and the maximum brake force readings themselves are not enough to say that a braking system efficiency has been achieved, however VOSA did point out that where the brake efficiency was not achieved, the NT could revert back to the old roller brake test methods, but I don't think they would have thought or meant you could revert back because you have no test weight?

So you are right to say that a decelerometer test is the correct method

Tyres

Modern tyres can be a mind field with their designs of treads, the best advise I can give here (common sense view) is where an element of doubt is present as to which parts of the treads are testable and which areas to look at in the centre 75% of the tyre, in my opinion, would be to look for any area of tread that is stepped, and disregard that area of tread for the 1.6mm rule, look carefully at other areas of the tread disregarding any sipes and look for the main tread sections with the TWI's in, and measure those areas.

The side wall(s) adjacent to the shoulder of the treads are sometimes marked with a triangle or the letters TWI, these point you to look in the treads at those points across the tyre, which show the main tread area to be examined. Not all tyres will have main grooves across 75% of the tyre width, which does not mean the tyre should fail, but if the tyre at each side of the central band is devoid or stepped and below the 1.6mm, this would be a pass and advise.