(b) If I would be linkvana I would reserve a fraction of the network for each customer so the other blocks are preserved.
I think that Linkvana alludes to that in their FAQs. While it would make it harder for the search engines to detect the complete network of sties, it would also have a potential drawback that your links would appear on a lower number of domains.
So you would end up making more posts on fewer domains, as opposed to less posts on more domains. I believe the latter is preferable in terms of flowing page rank.
At least, that is what I get from reading the FAQs.