Just trying to figure out what is the proper path with this spiteful phone - Sprint HTC EVO 4G LTE

So my girlfriend's EVO 4G LTE is a bit of.. well, I'm gonna say bastard. I'm not new when it comes to rooting devices but I've never had to deal with one that fought me so much.
I've been through this adventure many times with it, and it gets more complicated each time. Here is what's happened with it so far.
It was stock, and it was terrible. It lagged so much, despite factory resets, that apps like google maps were completely unusable. It got no battery life AT ALL, and was generally freeze-happy and miserable.
So then I s-off'd it and after learning all its quirks, installed recovery and CM 11.2 (I think? It's been a year)
This worked great! For.. a few days. The phone worked, we went on vacation to a roaming area where Sprint had no coverage. We come back, and only the data works, texts and calls do not, and the phone acts like it's not activated. Yay. All attempts to get it activated are met with failure, much to the confusion of the poor sprint person on the line. The phone just will not take, at all. (damn lack of real sim cards..) We tried recovering back to a backup from twrp of the stock firmware but that won't work either? It wouldn't register with sprint even on the backed up version of stock sense. The phone seemed dead at this point. Bricked radio?
So we give up. We bring it to the sprint store, and they do some magic to it, and its back to stock, slow and terrible, but functional. We don't touch it until about three weeks ago.
But, my girlfriend is tired, again, of the terribleness, and asks me to try again. Okay, sure.
I install TWRP again, so far so good. The phone is still S-OFF from last time, which is nice. I go to install CM11.2 from the files used from last time (the MD5 does check out correct) But, the phone complains about not having a new enough bootloader. ???? (So I assume sprint downgraded the bootloader in the process of fixing it. somehow)
I forget the exact numbers, but we can't install CM11.2, only 10. or 10.whatever. the newest 10. Okay, fine. I gather a new set of cyanogen files, gapps, whatever, and flash all of that, so far so good.
This time it works, and stays working! Yay! So I thought.
As it turns out, its on a very old version of android (so old that it goes into a special usb file host mode when usb is plugged in) and more importantly, the camera is TERRIBLE. I've read that the evo 4g lte uses a special ImageSense chip to boost camera performance, and that cyanogen 10 does not support it. I can tell, because the camera is absolutely terrible. The battery life also is terrible, presumably because it lacks optimizations. It had way better battery life on CM11.2 when it worked. Other then that, its working just fine, but the camera is just unacceptable.
So, I want to know what to do from here. I see a few paths.
plan A:
update HBOOT to a newer version to allow the install of CM11.2
(how do I do that? I heard you have to use an RUU, but which RUU? I read that there is some touchscreen driver that is an issue depending on what version you run)
(Once I install the ruu is it business as usual, install twrp and then flash cyanogen like normal? Will it do the crazy not activation thing again? was that a fluke??)
plan B:
update HBOOT, but to a much newer one that allows the install of CM12 or whatever the newer ones are. Presumably updating past the touch screen driver issue and not allowing us to go back to an old version.
(once again, what do I need to do that? Is this advisable? I quite prefer the idea of running a newer version. So if this is the better idea, I will do it.)
I thank you greatly for your help. I'm not used to it being this complicated. I've read a lot of things about this phone and it seems very conflicting at times. I just want to get straightened out.

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yeah, it's probably running like crap because the firmware is so out of date.
use this RUU http://forum.xda-developers.com/showthread.php?t=2653730
then re-flash twrp and another ROM if you want (i've been running cm12.1 for MONTHS) but she might be able to survive just like that. only fear would be staying vulnerable to security threats, but you've been living in that house this whole time anyway.
for a good while i was running this stock 4.3 with root and some xposed modules installed to get the cm functionality that i missed. but seriously, run the RUU, let it activate, then go under settings and update profile and PRL, fire up maps/navigation with GPS on so that it gets a good lock and its cache populated, then you should be good from there.

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Thank you!
Can I use the same utility that I used to flash twrp (that one-click thing that seems to support a half million phones) or do I need something special? This sounds very easy to do. Almost too easy, given that the story of this phone's life is that the easiest thing I've ever done to it was take it completely apart to replace its battery.

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never used the one-click stuff. flashing recovery via fastboot is simple enough.
get the adb tools, here's some easy instructions for that http://lifehacker.com/the-easiest-way-to-install-androids-adb-and-fastboot-to-1586992378
get the latest twrp from here https://dl.twrp.me/jewel/
then follow these directions to flash twrp https://wiki.cyanogenmod.org/w/Install_CM_for_jewel#Installing_a_custom_recovery_using_fastboot
if not installing cm or any other rooted rom (you want to root stock) get supersu from here http://forum.xda-developers.com/showthread.php?t=1538053 and use the twrp installable zip

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If you've gone through the trouble to S-Off, then fastboot flash the latest TWRP (3.0.2.0, I believe), which supports the new partition layout (extra 4gig of internal SD space).
From there, flash the official 651.4 developers version, go through the device setup which updates your PRL, then shut it down and reboot to TWRP and flash the latest CM12.1 and a proper gapps package (I use "stock", which is a bit much for some, but I like having everything available to me).
The one thing I do that most don't is to disable zRam and enable swap with my own script I wrote specifically for the external SD I formatted as vFat/ext4/swap.
After doing all this, I do not have the issues you seem to have, and the PRL seems to stay from the last install of stock Sense 5. Even with all the apps I have installed, it's still fairly snappy. Facebook/Messenger, and other memory hogging apps run smooth with no discernable lag, unless there's network issues.
YMMV, but look into using an app like "Ampre" to measure battery life and gauge if the battery is going bad or not.
Outside of that, there's not much left to do with these older devices that hasn't been done already.