The answer is B, it must be keeping it's own internal index. If you manually insert into the table, it requires you specify the SessionID. This would not happen if the column was set to auto increment.
What I've done before which seems to have worked ok is just used a negative value for sessionid (since the generated sessionid's are always positive and the column datatype accepts negatives this seems to work ok without having values get overwritten).
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.