Whether rooting voids the warranty depends on the manufacturer (or carrier if they are responsible for the warranty). Usually you assume it does unless you can find confirmation that it does not. But does a V20 have a warranty anyway? That's quite an old phone.
Developer mode on any device I've owned didn't offer any way to remove bloatware anyway. And no, that's not how a factory reset works: if you actually delete a system app, i.e. remove it from the ROM, a reset will not restore it. A "factory reset" just means "delete the contents of the /data and /cache partitions", and does nothing to the /system partition. Of course deleting stuff from /system requires root anyway. So I suspect you are thinking of some way of disabling stuff rather than actually deleting it (disabling is indeed undone by a reset).
I'd need to know more about what you mean by removing an app requiring removing dozens or hundreds of files to comment on that. Can you give me an example of what files you are thinking of? An installed app does typically have a number of files, but they are all in a few app-specific folders, and uninstalling removes those folders and their contents. Of course those folders are also not directly accessible without root, but that's what the system uninstaller does if you use it.
You can do nothing to the ROM without root. Literally nothing (though root by itself isn't enough: you need to mount /system rw as well, and some manufacturers have further protections against tampering).
Flashing a ROM is something you can only do with a rooted phone anyway (technically not correct, but only if you are very pedantic about what you mean by "rooted". Technically if you unlock the bootloader, install a custom recovery, and use that to install a custom ROM that is not itself rooted you haven't "rooted" the phone, but the first 2 steps are part of rooting and the third is a bigger change than rooting itself, so this is splitting hairs - if rooting will void your warranty, so will flashing a custom ROM. As most custom ROMs are themselves rooted anyway this distinction is moot in most cases.
In almost all devices all pre-installed apps are in the ROM (there have been a few enlightened developers who have pre-installed apps where the user can remove them, but they are the exception). Apps you install yourself you can also remove yourself.
Why they do it? Because they think it serves their interests. Remember what I said: there is no universally accepted definition of "bloatware". Samsung, to pick an example, often load their phones with Samsung versions of apps in addition to Google or stock Android apps (they are I think a little better than they used to be, but still do it). For most people one or other of those duplicates is bloat, but they won't necessarily agree on which. So why do Samsung do it? Because they think they can do better, because they think it establishes their brand identity, and probably because they think that people who are used to "Samsung" apps and UI are more likely to buy another Samsung. On the other hand I have a Pixel, which comes with various Google apps, almost all of which are "bloat" to me (and which I disable). But in both cases these are apps that serve the manufacturer: e.g. if you use the manufacturer's mobile payment app they get a cut of the payment (as well as data on your spending habits), if you buy media from their media store they make money, and many of them collect monetisable information about you when you use the apps. People tend to use what comes with the phone, rather than make the effort to decide what works best for them, so pre-installing works for them. Carriers add their own apps because they want you to use their services (sometimes removing standard Android apps that would not make money for them). And yes, some app developers pay manufacturers or carriers to bundle their apps, which means that the manufacturer/carrier does make money from adding the bloat (I've known non-carrier devices to come with regular commercial apps not only pre-installed but non-removeable and with even disabling forbidden - something that should only be done for truly essential apps, which EverNote, to pick an example of one that had such privileged treatment in the original firmware of my tablet, certainly is not).
To be honest I struggle to think of a single phone that is sold with "stock android". Some are sold with relatively light ROMs, but none I can think of are pure AOSP.
