Hi Petex,
With latex the density is directly related to softness/firmness and is a “comfort spec” like ILD and isn’t really important because your own personal testing will tell you more about how soft or firm the mattress feels in to you in terms of PPP than the density or ILD information. This is unlike memory foam or polyfoam where density is a “quality spec” and has little to do with the softness or firmness of the material. The type and blend of the latex would be more important information to compare it with other mattresses.
There could be some slight differences in the materials between the Halsey and the Seneca but it looks like they are at least close to the same. The specs you listed are all high quality materials and there would be no weak links in the mattress in terms of quality or durability.
A tape edged cover is much more common than a zip cover and wouldn’t make much difference if the layers are glued inside (and tape edging can create a tighter fit for the cover) but it also doesn’t allow the cover to be removed for cleaning (although some zip covers can be removed but not cleaned). Most foam mattresses don’t have side support and in general it’s not really necessary if the support layers are firm enough for sleeping comfortably but there are a few exceptions that do for those who prefer a firmer edge for sitting or who sleep with weight on the very edge of their mattress (see post #33 here).
The Seneca would certainly make a good quality choice if it’s a good match for you in terms of PPP.
Phoenix