You can almost always replace the battery, even when the manufacturer doesn’t want you to. As for flashlights, they typically come with easily user-replaceable ones, often even sold separately. Worst-case, you can get a AA or AAA flashlight and use rechargeable AA/AAAs.
Nah, cheap phones often have their bootloader unlocked/unlockable. Really happy with my POCO M5 running modified AOSP. Also, unlike every expensive phone nowadays, it has 3.5mm jack, SD card slot, and exceptional battery life for hiking/trekking (it survives 5-6 days as just a camera+map phone with all power saving on, in comparison people with flagships typically only last 2-3 days with the same usage and power-saving techniques).