I think this notion of an 'optimal' install is something of a chimera. Every installation is different, and the majority of forum users have experience of just the one installation. From what you have said (though images are worth a thousand words) your installation seems to be fine, the installers haven't done anything stupid.

If you are having vibration issues at the heat pump then it might be an equipment issue rather than a pipework issue. I have noise inside introduced by the circulation pumps, but only small amounts of occasional mechanical noise and the sound of the air being moved from the outside unit. The units weigh around 100kg so you wouldn't want any slight vibration to be converted into sound or transmitted to the rigid, fixed pipework, but they aren't going to move around like washing machines.

You ask a few questions:

28mm pipework should run as far as you can take it into the house - you need volume of water in the system to cover defrosts.

You protect the outside pipework and the pump either by having glycol so the water can just sit at very cold temperatures, or you circulate the warm water in the pipes to keep the outside parts from freezing – the anti-freeze valves are a fail-safe should you be unable to power the water circulation. You would not want to be dumping glycol.

Full bore valves are isolating valves...

I’d consider my system to be installed quite optimally… I have one anti freeze valve, no flexis, and over 20m of buried primary pipework.

I also don’t have any hydraulic separation, and I have 400mm clear to the rear of the evaporator.

Posted by: @harriup

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28mm pipework should run as far as you can take it into the house - you need volume of water in the system to cover defrosts.

Yes.  Where would the optimal position and method of stepping down?  I assume nearest the unit, so at the unit's inlet/outlet - but not sure how.

You protect the outside pipework and the pump either by having glycol so the water can just sit at very cold temperatures, or you circulate the warm water in the pipes to keep the outside parts from freezing – the anti-freeze valves are a fail-safe should you be unable to power the water circulation. You would not want to be dumping glycol.

Thank you.

Full bore valves are isolating valves...

From the BES component diagram in the OP (relevant section below), it shows 'isolator valves' and 'full bore valves' - though, having just looked at it again, I've just noticed the isolator valves are inside the property:

The isolator valves I mentioned describing my install are Inta lever ball valves - I assume these should be full bore to maximise flow rate and be positioned as the primary pipework exits the brickwork outside?  Is there any value in having a second set at the unit's inlet/outlet too?

Posted by: @rhh2348

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What's the optimal / minimum length and shape of flexihoses (and why, if not for best performance and least vibration)?

I'd appreciate anyone's thoughts on this, having read in one or two posts (I now can't find...) that longer hoses can reduce/eliminate vibrations travelling through the primary pipework elsewhere.