What happens with contacts that leave flows, and then is imported again?

Short answer: Flows remembers the contacts last position.
Long answer: Since Flows uses Segments for adding and removing contacts dynamically a contact that doesn't match the segment criteria is automatically removed from the segment, and thereby the flow. But flows keep track of which step the contact was at when it left the segment. So if the same contact is added to the segment again, he or she continues their journey through the flow.

Why is the sum of 'contacts here right now' and 'Contacts passed this step' not the same as the total amount of contacts in my flow?

What happens if I remove a step that has steps below it?

If the step you are removing has one or more steps below it, these steps will also be removed.

Can I add contacts in my flow to an Advertising campaign?

Yes, but right now it requires a bit of a workaround. Here's how:

1. Start by creating a Sales campaign in Upsales. (We are going to select this in the advertising campaign later on.)
2. In your Flow add an Action step
   1. Select the Action type called Update company
   2. In the dialog named Update companies that appears when clicking on the action select the sales campaign you just created in the field called Add Campaigns.
3. All companies reaching this step will now be added to that sales campaign
4. Go to Advertising
5. Create a new Advertising campaign
   1. In the card called Targets click on Account Based Marketing
   2. In the dialog that appears, add the Sales campaign in the Campaigns field.
6. Configure the rest of the advertising campaign settings and click Save
7. That's it, all contacts reaching a certain step in your flow will be added to your advertising campaign.