If you have a dual modem router that was used by loads of devices running loads of applications, the network sessions generated by those applications would be distributed across the dual modems in a load balanced fashion and you would end up using all the bandwidth available on both modems.
If you are a single user uploading a single file to a single website (which is typically a process that maintains a single IP session), that upload (and the underlying session) can only ever use a single modem - unless you use bonding. Bonding requires a remote end point - either SpeedFusion Cloud or Fusionhub (hosted in the cloud / or a datacenter) or another Peplink device hosted in a datacenter.
Bond no - not without SpeedFusion, Load balance yes.
Yes but Peplink define Hot Failover as packet level failover within a SpeedFusion connection. Without SpeedFusion Cloud / FusionHub you simply have failover, which is where the router detects there is an issue with a modem so stops using it and outbound traffic from any device that was using it will be sent via another healthy link. (good comparison here by 5G Store)
In real terms, if using SpeedFusion Bonding (and so hot failover) and a WAN link fails, traffic is rerouted so quickly that a voip call, or video call, or a file upload will typically continue unaffected. When load balancing, its very likely that all of those things will stop when the WAN fails and will need to be restarted.
Yes you can do this, or use the WIFi WAN connection within the SpeedFusion connection too.
Although 5G coverage is improving every day, every deployment I have seen so far is NSA (or Non standalone) so is still using 4G infrastructure - mainly for the upload role. if your focus is on upload bandwidth 5G doesn’t really offer much of an improvement there at least not yet until full 5G Standalone is rolled out. 5G SA is still in pilot stages I think in most countries.
If I was you, I would likely stick with a CAT12 Transit Duo now, then upgrade to a 5G device later when coverage is more complete. Dual modems will give you much more coverage and less hassle when travelling compared to a single provider - particularly if you will be on 4G most of the time.
The BR1 5G has been released in low volumes for Partner and specific project testing. Its not commercially released yet.