Well, the "Explorer Edition" units haven't gone on sale yet. Right now only developer units are on sale, and those are strictly for developers who will contribute to the kernel, firmware, drivers, and shell. But I'm sure they will be willing to process a refund for you if your device hasn't shipped out yet, otherwise you'll have to return it minus the cost of shipping.
They ship Manjaro ARM because the people behind PINE64 are manjaro fans and they think that Manjaro has a good business setup. It would have been much nicer to have PostmarketOS shipped stock.
It does "run", but it's not really running Linux. It's just android with a halium layer running Linux software on top. So when android support ends, which it will seeing how they've not been great at software support, your "Linux" build will also no longer get updates. That's why Mainline Linux is such a big deal, as even if support ends, you'll still be able to get the latest Linux kernel and distributions can continue basically forever supporting the device.
The problem is that it's not 1-1 with X86, especially in regards to graphics capability (The GPU is really the weak link on the current PinePhone). The GPU for example uses Tile-based rendering, which requires software optimizations to work best... Plus the GPU is just plain slow. It's a first generation ARM Mali graphics core intended for OpenGL ES 2.0 afterall.
As for the headset, i would suggest looking into a pair of bone conducting bluetooth headphones, such as the Aftershockz, as they have a built-in microphone and also are open ear design as they obviously aren't normal headphones. So you can use them in the car or anywhere, and keep them on wherever you go. I personally just bought a pair and used them already to listen to some music off my PinePhone.
With a Bluetooth headset i think it would be okay for calls, and infact the keyboard might make it much nicer for texting.
As for notifications: You can't see notifications from apps on the PinePhone anyways unless you disable suspend, as the crust suspend firmware only detects wakeup events from signals sent by the hardware. So while the modem can send a signal to wakeup the device when there's a call or text, and the RTC (Real Time Clock) can wakeup the Phone when there's a alarm going off, there's currently no solution for waking it up when say you have a new Telegram or Matrix message while in sleep. More than likely people will figure out a way to solve that in the future with custom wifi chip and modem firmware that can handle push server wakeup events.
With that said though, the 6,000 MaH battery this keyboard addon will have should make up for that. You could disable suspend and have all the apps constantly watching for new messages and sending notifications, and probably wont have to worry about battery life even still. This is exactly what i will be doing personally.
The only problem i forsee is how big and bulky this is, meaning it will probably not be feasible to use one handed, and it also will be heavy in a pocket.
I sure think so. Less a killer though and more of a real continuation of it. Not to say the Gemini, Astro Slide, etc isn't continuing it, but Planet Computer's devices run Android and not mainline Linux like the PinePhone does (sure you can run Linux, but only with an android kernel).