Hi knicoletto,
The deeper layers of a mattress are generally not the weak link of a mattress so underneath 6" of latex the 1.5 lb polyfoam wouldn’t generally be an issue or a weak link unless you were in a heavier weight range.
I would generally suggest 1.8 lbs as a minimum for a comfort layer (unless it was two sided where 1.5 lbs would be OK) or you were in a very low budget range but using firm 1.5 lb polyfoam underneath thicker comfort and transition layers can be an effective way to lower the cost of a mattress without compromising durability for most people.
If it uses talalay latex made by Latex International and the layers have a specific ILD such as 19, 24, 28, 32 or any of the others listed by Latex International here then it would be blended. If the latex is N1 - N5 then it would be 100% natural but wouldn’t have a specific ILD. Generally (and hopefully) the manufacturer themselves would be the most reliable source of information about the materials they are using in their mattresses.
Both mattresses use 6" of latex so from a “raw materials” perspective they would be roughly comparable if both of them are using 100% natural Talalay. The type of cover and quilting materials can also make a significant difference in the cost of a mattress. I would also make sure you were comparing “mattress only to mattress only” (not a set) so you are making apples to apples comparisons because they may also be using different types of foundations or box springs. If the Lebeda is using a box spring that flexes (rather than one that has a rigid non flexing surface) then it would also be part of how a thinner latex mattress feels and performs so they would need to be purchased together or the mattress you take home will feel different from the one you tested. With a rigid non flexing foundation then any suitable non flexing foundation would generally be fine (but always check with the retailer or manufacturer if you are using a different foundation from the one they recommend or is part of the set to make sure it meets the warranty criteria).
I certainly don’t agree with this at all. They have a different “feel” and performance but are very comparable in terms of durability. You can read more about the different types of latex in post #6 here.
It seems to me that you have eliminated your worst choices and are looking at final choices that are all between “good and good” so which one is “best” for you when there are no clear winners would depend on all the objective, subjective, and intangible criteria that are the most important parts of your personal value equation. Lebeda can also make custom adjustments to some of their mattresses outside of just exchanging them (within 730 days) but I don’t know if this applies to the mattress you are looking at.
Phoenix