Hi lumy and welcome to The Mattress Underground 
glad to have you here.
It can be overwhelming trying to find a new mattress, and you are one step further trying to find a suitable mattress for your mom. Any mattress will be unique to the sleepers using it based on their stats (height, BMI, sleeping position(s) and any underlying health conditions) and your PPP(Posture and alignment, pressure relief, and personal preferences). You might want to read the Mattress Shopping TutoriaI as well as theMattress Specifications You Need To Know and the Mattress Durability Guidelines to get a better idea of materials and components and how long they last.
[quote] I’ve ordered a BB Signature (Firm), which she found too “memory foam hot” after one night. And also not firm enough.
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[quote] Bite the bullet and just get another Nest Owl in Firm.
Get Brooklyn bedding’s CS to exchange for something else, but the next firmness level should be Plank, which I read sometimes doesn’t hold up its firmness? She’s more or less lightweight so it might be fine.
Drive 1.5 hrs to Santa Cruz to check out Bay Bed, but I don’t know how much their mattresses will cost, might end up getting something not that much different from the Owl (in terms of price)
Find the same beautyrest bed she slept on for years, but I’m pretty sure the Asian models are different, they don’t even have the same sizing standards
Random Amazon actually firm non-hybrid innerspring mattress?[/quote]
It sounds like the Brooklyn bedding Memory foam sleeping too hot, which is a common issue for some many consumers due to the its viscos nature of memory foam as it forms a tight cradle around the body and traps the body heat not allowing for sufficient ventilation – as it conforms to the sleepers body, it causes perspiration to ‘pool’ close to the skins surface, which heats it up and can be uncomfortable to sleep on.
You did not say what size bed you need for your mom’s bed in the guest room but either way, your budget is on the low end, for a durable and supportive mattress that does not sleep hot. I would caution you that you don’t fall for basing your purchase on any reviews and don’t fall into “randomly” getting the lowest budget (usually inexpensive cheaply made) bed from amazon, as these can ‘break down’ prematurely and need replacing. I would probably discard this approach unless you manage to find out the specifications so you can compare the quality of the materials and components to the mattress durability guidelines here. However, being in your guest bedroom even a very low budget mattress may last a little longer so I’d look at the pros and cons for this choice basing it on what is important to you based on your personal value equation as described by Phoenix.
It may also be difficult, as you say, to replicate the old Beautyrest that your Mom enjoyed, as they are constantly changing the components and compositions of their mattresses.
The Brooklyn Plank may be an option, the basic version is 11.75" Tall, 4 Layers you can read in Post #3 here that some consumers have found the plank to ‘soften up’ once it is broken in. These are good quality mattresses but it comes down to how firm she likes her sleeping system.
Bay Beds appears to offer Hybrid Latex mattresses with innersprings, and while it’s a bit of a drive for you, may be worth first calling to check out to see what they carry, and how their prices compare. As you know BayBed is one of the. Trusted members of this site and have an excellent track record of finding “out of the box” solution for any customer need. I’d definitely give them a call and also see how low the TMU discount can bring the pricing down.
[quote]…She has slept a few nights on my Nest Latex Hybrid (Firm) before and liked it (but this is currently a Medium since I prefer that) …
Bite the bullet and just get another Nest Owl in Firm…[/quote]
The Nest Bedding Owl may be a bit higher than your budget (though they typically have sales and there is a TMU discount which may align more closely to what you want to spend) but has the benefit of your Mom already finding them comfortable. As many large retailers like Macy’s and Walmart carry these, it might be more convenient to try them out and ‘dial-in’ your Mothers’ preferred comfort level.
If you are considering online ordering, you can review the offerings of theTrusted Members of the site; they all have great experience in matching sleepers to their ideal mattresses; they are fully transparent about construction and components, and if you can provide a little bit of info on your Mom, they should be able to guide you through their options.
You may want to check out GhostBed (among the Trusted Members as well) they offer some great deals not just two weeks before a major holiday. Plus, they have an exclusive with Affirm for 3 to 5-year 0% APR when competitors are only 18 months to 3 years max… so it’s helping with this new economy as more people look at financing to invest in sleep. As they are more and more consumer friendly and accommodating this is reflecting it their popularity and market share for top bedding companies. There’s nothing wrong with this just wanted to give them a little more visibility)
I hope this helps,
~ Basilio