I'm no expert but what I'll share what I do know in response to your observations.
Firstly, IP numbers. You can't obtain anyone's email address from detecting their IP number. You can detect someone's location to their town or city, but not their street address. You can obtain some basic information about the computer or device currently used to access the internet at that particular time ? phone or computer, Mac or Windows, the browser and the like. You can't find out the identity of the person, or age, or their street address or their email address.
There's a myth that people have a fixed IP number, but most home users have a 'dynamic' IP number assigned by their internet provider when they connect to the internet. If you don't disconnect completely from the internet ? that is, switch off the router and/or pull the plug on it as opposed to just shutting down the computer ? your IP number probably won't change, it will just keep renewing. So, if you want to change your IP, unplug from the outside world for a couple of hours or overnight. IP numbers are not like phone numbers. They aren't assigned a particular computer or a particular person. You can pay for a 'static' IP but most home users don't do this, some businesses might.
Tracking cookies are what's responsible for those annoying e-commerce ads that seem to follow you around online. Typically they've come from a site you've been on earlier, and almost always from an online retailer. So, you do a bit of virtual window shopping and then whatever you were looking at shows up later in an ad, on a webpage from some other site. That's tracking cookies in action. The way to stop it is to delete your cookies. You can't really avoid them if you shop online. Some stores are worse than others.
So.... spam email. Yes, I've had emails trying to sell me viagra and all that garbage, Russian brides or whatever. Email programs are so much better at filtering out this stuff automatically so I don't get to see it as much.
Having said all that, blaming the ex partner for suspect emails doesn't seem credible. Many porn and cybersex addicts become pretty adept at covering their tracks to avoid discovery although some may be complacent in the assumption that their wife knows nothing about computers and anything that does look a bit suspect is explained as "it must have been a pop up" or "a virus" or "so-and-so at work sent me a link and I had no idea what it was until I clicked on it". My husband knows he'd never get away with excuses like those.
Yes, there are legitimate reasons why you might get junk email or you get stalked by ads for shopping sites you've previously visited, or travel destinations in your case, but many partners get fobbed off when they do come across something questionable. Remember ? don't be ignorant, knowledge is power. 8)