Shortest Path for Certificate Network
Quoting SEJPM's (now removed) comments:

For the convenience of any answerer, I've rendered the graph and answerers may use it (and any derivatives) at their will. If required I can also provide vector graphic versions of it.

In this case one could argue that A issued both bob's and alice's cert indirectly and thus either party seems to trust A to certify CAs and thus they trust each other's cert. (But this doesn't really sound convincing to me -&gt; no answer)

Thanks to @SEJPM for adding the graph. With what you stated you were give, it would not be possible for Bob to verify Alice's certificate.