Welcome to the forum!
Firmness in a sofa (and ease of getting in and out of it) has as more to do with the seat decking and underlying spring rates than the cushions themselves. In the brands I sell in my store, I find Hancock and Moore as well as Leathercraft meet that criteria the best. Other makes I carry seem to have a higher 'sink' factor.
To keep the firmness, stay with standard Qualex cushions or Fiber Fill, don't go for and of the optional down cushions. In addition, companies like H&M will modify the sofa for you if you ask (and make it even firmer) by changing into higher density batting on the cushion wrap. One has to be sure this is what you want however, as when you ask for extra-firm, you get it.
If you're dealing with MS, you may want to consider a sofa that has power recliners on each end. I'm fairly confident that a power lift mechanism could be fit into at least one of the end reclining units. This makes it extremely easy to get in and out of the piece as the seat of the unit comes up to meet you from a standing position, and you also would have the benefit of a power recliner via remote control. When you want to get out, the power mechanism can lift you into a standing position as well.
H&M is the most receptive to doing custom mods of all the companies, so if thats something that interests you PM me and I'll make some phone calls to find out what we can do.
Any leather should easily go 10 years or longer. I'd be disappointed if they didn't go triple that. You will not likely 'wear out' any hide, rather it becomes an issue of lifestyle and use as to what hide would work the best. The Capri you mentioned is unprotected leather, a very fine hide, but susceptible to staining and absorbing grease, etc. If you intend to use the sofa casually and have food and drink around it, then that's a poor choice due to the possibility of having numerous stains on it. For hides that can take more abuse and sluff it off, look to Finished leathers in the lower grades.
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