Dear chicken friend.. we are our own worst critics and it's GREAT that we feel convicted by our subconsciousness! 
 
Did you send the second bird in for necropsy.. or attempt to do one yourself and look at the liver and rule in or out that possibility? There is NO such thing as coincidence IMO.
Understand that I have many, many
 known so called toxic or poisonous plants growing throughout my property including morning glory, tansi ragwort, buttercups, bracken fern, mushrooms to the nth degree, the list goes on and on. Keeping chickens, goats, and pigs was extra scary at first. Through research though I found out something can be referred to as toxic or poisonous if it cause mild upset stomach, or headache.. or EVEN if it causes euphoria! Most of the things referred to in this matter will NOT be an issue UNLESS the birds are locked in with nothing else to eat. Some things are of real concern like Oleander IS actually deadly fast. A chemical spill (rat poison) or botulinum could also cause quickened death like this. Despite tomatoes being in the nightshade family my birds will mow the whole plant to the ground. They are NOT the same as Deadly Nightshades. I would NEVER take the risk of knowingly keeping oleander around, though we liked them as a kid.. even though the animals appear to munch on the bugs attracted to and other plant life around.. leaving the "toxic" stuff to flourish another day. Ugh, my neighbors flowers are always spreading over here.. most recently Foxglove. I am VERY happy to report though that they have decided to quit using herbicide along our mutual fence for the sole purpose of being kind to my animals! But NO, I don't think it was morning glory that caused your issue..
I wonder 
where are you located and did YOU 
read any of the 
ingredients labels on the feeds they had to offer.. or 
review the thread posted earlier that has accumulated all pertinent information on MANY of the feeds available to us.. to see which one might be BEST suited to your liking and kinda know what you're looking for? Always and I do mean 
ALWAYS.. get a second opinion on ANYTHING told you by a feed store employee.. most of whom often have ZERO actual animal experience and even those with the best of intention still give some seriously HARMFUL advice! SOME feed store owners and managers MAY be a bit more knowledgeable and more invested than a minimum wage employee. And some BYCer's even 
unintentionally give misinformation or learn something new but cannot change prior posts! 
 
Tractor supply and Chewy both offer vegetarian formula chicken feeds. The Purina flock raiser I use is vegetarian despite NOT being touted or advertised on the bag. I actually would have to SEARCH for a chicken feed with animal product in it. I use to laugh when I saw "vegetarian" fed hens and thought they never saw the ground, outdoors, or light of day if they never ate a bug.. but 
now I 
do comprehend that some animal feeds are being made with with deceased animals that were not slaughtered but died of other causes.. AND despite it being against the law in many states they do NOT enforce the law effectively from state to state or federally and brands still crossover.
Realistically though.. animal byproducts cost more than vegetable byproduct.. Companies are out for their bottom dollar and so are many family budgets, it's just not cheaper. Even at Walmart.. I couldn't find chicken feed WITH animal byproduct of any kind other than oyster shell which is clearly labeled as such.. So, which feed did you pick up? Is it already included on Kiki's list? Is it really not vegetarian? No ugliness meant, just conversation and curiosity! 
 
The gals you mentioned are known more for reproductive disorders than anything else (kidney, liver, etc).. But, 2 suffering the same fate so close to each other contraindicates reproduction condition AND a necropsy could rule that in or out as possible cause.
Since you said you'll be trying other breeds, here are a couple of charts for comparison. Just don't believe everything you read as many birds are better on paper than in person, the rarity of many breeds is outdated information, and despite breed tendencies ALL birds and people are individuals..
Chicken Chart
Breeds of Chickens
https://livestockconservancy.org/images/uploads/docs/pickachicken.pdf
If you wan't something that thrives under YOUR conditions then breeding for it is THE way to go. 
Land race.. is a "term" that is HIGHLY over rated in MY experience and even the so called "land race" breeds HAVE been selectively bred.. I PROMISE nature don't give a bleep about whether or not a Cemani/Svart Hona has a white toe nail or pink inside his mouth. If you WANT high qualities you MUST select for them. Why are so many land race breeds straight comb when that's the absence of a gene? And how are there so many Swedish land race breeds?? Svart Hona and Swedish Flower Hens? Just questions from my overthinking brain, sorry! 
 
Game on the other hand.. holds true to it's purpose more so, as far as can tell, than land race.
Also, please understand that there ARE other "old school", "old timer's", natural, and/or complete free range ONLY type keepers and threads on here you might turn up results using some of those terms! 100% my way and what I'm suggesting are only ONE way and what what works for some in one environment may or may not work for others even in the same environment. There is NO perfect way.. just what works and what doesn't. YOUR mileage may vary, and that's okay! Make the best choice you can with the information you available at the time. If it isn't working or you learn something new, switch it up!
Yes, your birds are gonna be miffed about the change in pay out as they are creatures of habit. They WILL adjust before long, just stay strong chicken mama!
No need to ferment your feed if providing a correct ration. I did only fermented feed for a flock of 82 for 2 years and didn't save a dime, increase health or anything else touted for my already healthy flock. I wasted a lot of time and energy, and I had tons of fun running different experiments. 

 But the end result when using a formulated ration you can get similar benefits just by wetting.. also.. if you're doing FF well, no need to drain but here's a little link.. (this link gives me flack so I'll figure out how to make it functional, sorry!)
file:///home/chronos/u-5cc07996839ed7db41ca2bf9246136425b20a5ce/MyFiles/Downloads/60378-Article%20Text-111356-1-10-20101001%20(2).pdf