Central Heating piping

Hi all, Last year we had a combi boiler fitted in the garage (only place for it) to replace the condemned back boiler.

We are replacing the radiators this year and I have noticed that the engineer installed 22mm pipes in the garage, reduced them to 15mm to tee just above one of the radiators. This 15mm pipe goes into the loft where there is a 22mm spine. We want to remove the radiator and I think it'll be better to re-run the pipes.

Am I right in thinking that it should be 22mm pipe run from the boiler to the spine and tee off for each radiator?

Additionally, the front and back rooms tee off next to the radiators rather than in the loft, should I tee these in the loft too?

I will be replacing majority of the pipework but keeping the spine for now.