I agree with 1,2, and 4. However, doing everything together feels very unhealthy to me.
If I think back to the happy couples I’ve known, it’s pretty normal for this to start to happen with older people, but older people are already a bit co-dependent because of physical limitations (as you mentioned above with this couple).
When you’re younger and have the ability, I think it’d extremely important to maintain a healthy balance of doing things together and being your own person.
If you do everything together, at best, you soon run out of things to talk about — that person was with you throughout your entire day, thus there’s nothing to inform them of.
At worst, if that person is suddenly no longer around (maybe s/he suddenly dies) your entire world has just become inconceivably lonely. You have no identity without that person. You’re not a “me” or an “I” because you’ve been an “us” or a “we” for so long.
It’s normal for relationships with others to eventually end (even if only through death). The only one that will ever be around our entire lives is our relationship with ourself. This makes nurturing your self relationship that much more important while you’re in a loving and caring relationship with someone else.