you can not add an adult application - they have to be separate
for the passport, try this
1. Go to this link https://secure.cic.gc.ca/ClientContact/en/Application
2. Select 'Add a document to your application' and continue to the form
there is generally a standard guideline for the subject (and/or attachment) in the oath notice email or the clerk provides instructions. If there was none - you should be good
This is rare - have heard of couple of instances where in-person interview was conducted and they had the oath ceremony scheduled at the same place for later in the day
it's completely random - nothing to do with online vs paper. My partner and I both had an online application, mine got processed in 5 months and my partner will be taking oath this week after almost 10 months of application date