Ceiling – Fitting an ornate ceiling rose

Is the 10amp chocbloc sufficient for the load of 5* 42w halogen lights?

Yes, 5 x 42 W = 210 W, 210 W / 240 V = 0.875 A

Is the 'manual' twist of 5*neutral 'blue' cabling from each arm safe twisted into a single 10amp chocblok connector?

Five is a lot. I'm surprised the fitting didn't provide a better way to unite them. The main issue is ensuring that the connection is tight and that enough of all wires are clamped under the screw. If in doubt you can use multi-way connectors such as Wago push-fit or lever-clamp connectors which have a separate position for each wire.

Can I increase the risk by double insulating say the single Brown arm combine to live supply?

I'm sure you mean decrease the risk but I don't understand what you mean by double insulating.

For general lighting applications there isn't any need to apply insulating tape so long as everything is done correctly.

The live and neutral wires are adequately insulated and you shouldn't trim more than the minimum insulation needed to fully insert the wires into the connector. After doing so, there should be no exposed copper, the screw heads in the connector are protected by the plastic within which they are recessed, there should be no danger of metal to metal contact.

Any metalwork in the light fitting that can conceivably come into contact with people should be connected to protective earth.

It looks as though you could actually pull this off without drilling/cutting extra holes. Yes you just drill the holes. Keep them up from the bottom, or from the top about 2" so no fasteners from the floor or drywall or plaster is hit.

I have used 3/4" bits for easy holes to pull wire through and up to 1 1/2" bits for more difficult fishing, as in threading a needle from 2 ft away. If the threading kit you speak of are fiberglass rods you connect together, these are perfect for this.

If the location of the new holes are relatively accurate how they are oriented to the joists, with care and a few drill extensions you could drill so the end of the bit can be used to pull a wire back through. It will take extra patience to do so but it can be done.

You could also use to fish tapes, or your threading kit, I presume, if you had help to send one out through the blue hole and cross it with another from the other holes, or you could go at the angle in one shot. I would assume you would pull the box at the blue location.

Here is a little sketch using you drawing to show what I would do.

From the blue hole drill 4 holes in line as best as you can with the other lights. Feed 2 wires to the fist light, one is supply, the other goes to the next light, after that it should be simple using the blue location to redirect the wires you pull to finish the remainder in that area. The rest you will need to thread.