How many in total? Usually you put up to 8 together by manifolding them and using reverse-return. No need for balancing. the only reason 8 is the limit is thermal expansion of the array. nothing really stops you from adding more 8-panel arrays if you put some expansion loop between the arrays. I'm sure there is something fancier for expansion.

Maybe a schematic? Why do you have a set of 2 and 8? Is that a roof area limitation?

Yes, it's a roof limitation.
I think 8 (or 10 according to others) is the suggested maximum number of collectors connected in parallel in one string(and provided inlet and outlet are in opposite sides, when at the same side this number is lower).
I am not aware of any limitation regarding the number of strings.
Interconnecting strings(sets) with reverse-return piping only(without use of self flow regulating valves) is not enough since each string demands different flow rate(due to different number of collectors).

The question is whether by installing self flow regulating valves at the inlet of each string, interconnection of strings with reverse-return piping turns out unnecessary/redundant? It seems to me to be so, but as I stated before I have seen both together in some publications(coming from well known manufacturers, organisations etc.), so I wonder whether there is some logic in it.

p.s. the number of strings in the sketches is only indicative, actually there is around 15 of them.

With balancing valves, direct return will work equally well. Use self-balancing valves where you set the flowrate and they self-regulate.

Manual balancing valves never get se right and are a pain to actually measure and some joker will adjust them "if something doesn't work" or will use them as isolation valves.
Rule of thumb where I live is 1gpm per collector. but may differ based on climate and what use the water is for (temperature needed).

You can add a thousand strings in parallel. there will be other technical and economic reasons to not have so many, but hydraulics doesn't' prevent you from doing it.

Depending on the manifold size in each collector, you could run both pipes from the left side of the bottom array to the right side of the array above and so on. then you wouldn't need balancing valves. But with the total flowrate of 15 of those arrays, the collector manifold may be too small.

Take into account this is glycol when sizing the pipes. Some designers underestimate the added pie size for it being 50% PG or whatever you use and assume it is like water.

Thanks.
p.s. Here the rule of thumb is 50 l/h·m2 of collector

The only logic would be that if the strings were of a similar number of units, then the reverse flow would mean that the pressure and flow across each set should be similar and hence you can use the same size flow control valve at each one and probably set it more or less at the same pressure drop.

There will though be a lot less flow through your two unit string than the 8 unit version, basically 2/8, in order to arrive at the same temperature differential. So in that case the reverse flow version is a waste of pipe.

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If each array would have the same number of collectors, you wouldn't need balancing valves with reverse-return.
Unless you use pressure-independent balancing valves, the manually adjusted ones are terribly inaccurate. You would make things worse.

I love the way different industries call the same thing. PIBVs to me are simply self contained flow control valves / fixed Differential Pressure valves which just look at flow or differential pressure form one side to the other no?

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Pressure-independent balancing valves do NOT have constant pressure drop. They have constant flow. IF the system pressure increases, the pressure drop will INCREASE to maintain the constant flow rate.

Badly worded. What I meant was constant DP across the thing (AHU, Collector etc) which is the same as constant flow.

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