Aptera appears to be locking into a single source for DC fast charging, Tesla. ...what assurance would an Aptera owner have in long term access to the Tesla's network and what would be the surcharge to use the network?
First off, Aptera's management loves the Tesla connector because of its simple, elegant design: Unlike all other charging standards, the Tesla connector can "pump" both AC and DC current. When it comes to DC charging standards the only real difference between the various formats is the shape of the ports and plugs - and Tesla's port is only half the size of other DC ports. As Tesla was the first US manufacturer of EVs - and the first to create a charging network - they created their own connectors. Nissan, the other provider of EVs, used the Japanese ChaDeMo format (although Nissan's coming EVs will have the CCS port).
At the current time only Tesla vehicles can charge at Tesla Supercharger stations in the US, Teslas can ALSO charge at both ChaDeMo chargers and CCS1 chargers - they just need to use adaptors. If Aptera does go with the proprietary Tesla port on its vehicles they, too, will be able to charge at any charger, so long as they have the appropriate adaptor.
Tesla has recently announced that they will soon be adding CCS1 connectors at some of their Supercharger locations so that non-Tesla vehicles will be able to charge on their network. To be eligible for Federal infrastructure funding, Tesla must offer non-proprietary-format charging. Non-Tesla drivers will need to activate the dispenser using a Tesla app on their phone and the purchase will be charged to whatever credit card the app has on file. Tesla is currently using this system in Europe and non-Tesla drivers are paying a few cents more per kW than are Tesla drivers.
It's an evolving situation and EV charging over the next 3-5 years will be a different animal than it is today: But, rest assured, it's an evolution of improvement.