L: Can there be single links (instead of double linking)?

Yes. We highly recommend double-linking on general circle level (between department circles and the general circle). Yet, on sub-circle level or below, single links can be enough. Double linking is also useful to create redundancy.
Remember that double-linking is a very effective strategy to make sure information flows and power is shared between circles. Yet the same needs can, under some conditions, bet met with a different strategy, for example, liaisons, delegates on-call etc.

For more information, check out our handbook Many Voices One Song section 2.4