why are my heating pipes routed like this?

Can't see a good reason.

No harm in doing so. Don't worry about balancing - it's never wrong to oversubscribe the potential flow then throttle it back at each rad with the local balancing valve. Undersubscribing the flow is where you get problems...

I suspect the 28mm pipework is a relic of a much older boiler. We have a similarly "odd" bit of plumbing in our system after an old floor standing boiler was removed and a new boiler fitted in a new site. Our system now has a 22mm--28mm--22mm section like yours.

Tim

Yes, probably remnants of gravity flow in 28mm to the cylinder with a pumped heating system from the boiler in 22mm; converted at a later date. That was common in 1973.

You should keep the cylinder return separate and only connect it into the main return after the upstairs and downstairs heating returns have joined; you'll otherwise get a nuisance heating flow in summer. The only illogical bit seems to be taking the heating flow from the valve downstairs and then taking the upstairs flow back up.