The condition of the plant makes id a challenge, but it looks like it might be Coprosma repens.

You are right! It's a Coprosma repens 'Variegata'.

In the past few months, it's declined dismally from the full-bodied shrub with bright green leaves with crisp yellow centers (see below) to its current condition.

I'm wondering if the plant is beyond its years (it's over 20 years old) or if a pest, disease or some environmental problem (such as poor soil, watering or lack of fertilizer) exists.

Before I buy a replacement Coprosma (same variety), I'd appreciate your diagnosis.

Question #2 is even trickier! Gary, I really don't know the general life span for coprosma. But if the decline happened that quickly, it does not point to an old age issue.

I don't know which came first, but the plant has been heavily pruned. If some established plants get moderately-heavily pruned at the wrong time, the undergrowth can suffer from the sudden change in heat and sunlight. Then that damage might get removed, and the condition cascades from there.

I believe coprosma is more sensitive to summer pruning in that regard. When I grew some ('Evening Glow') two years ago, I recall making a firm mental note to prune them in late winter, and not at all during the summer. But I don't recall where I picked up that information or if that applies to C. repens. Or if it was a factor that I am not a shy pruner.

There is new growth coming from the lower stems. If the root system is healthy, you could choose to cut it back to about 10-12" and see if the fresh growth takes off, pinching as it grows to help the plant bush out. Riverside is still experiencing some hot temperatures, I believe, and will have some colder ones coming in the next few months. If you do prune back, you might want to wait until late winter, and/or temporarily provide some protection for the tender growth. Something like a piece of shadecloth draped over some small stakes would help.

My coprosma's roots are almost next to the wall of my house and the trunk angles out toward my patio (see photo below).

The plant had been growing so wonderfully for many years, I didn't want to risk killing it by digging it out and replanting it in a more-vertical position. Instead, to keep the branches from extending too far out over my patio, I'd prune the plant back several inches every year.

Since my coprosma now looks too far gone, I'd prefer to replace it with a new one (same variety).

The camera angles in the earlier photos made the plant appear shorter than this side view. You shouldn't have too much difficulty finding a replacement; I believe they are still widely available. Often it really is best to revitalize the landscape with new plants, but I don't always recommend that simply because many homeowners become very attached to the plants, and are loath to remove them for even structural damage reasons. I'm a yanker on the other end of the spectrum, because I'll not flinch removing healthy plants, for no other reason than I need room for something else. ;)