Re: Plaster repair in non-visual locations
Prep the broken edges and exposed lath with bonding agent, and fill the voids with basecoat plaster like structolite.
Patching plaster from the big box store is pure gypsum (plaster of paris) and has a host of shortcomings that recommend against it. Brittle; won't bond to old plaster; no working time. The original white coat plaster in old houses is primarily lime with a small portion of PoP added as a hardener. That's why lime plaster has a longer working open time and bonds to the basecoat.
The best thing for a homeowner to fix small cracks with is Durabond 45, a setting-type drywall compound that is mixed from a powder; it sets and dries faster than ordinary mud, but the "45" means it has at least 45 minutes of open time at normal temperatures. It has additives (a glue) which help it bond better to all substrates, is moisture-resistant when cured, (great around showers and tubs). It cures very hard and is not sandable, but it can be scra=ped off to level as needed, and top-coated with the sandable type (Easysand ) or ordinary green-lid wallboard compound for a durable repair. If you want to go deluxe, use bonding agent and fiberglass mesh tape on the cracks and the results are even longer-lasting, unless the cracks are still moving.
Remove not the ancient landmark, which your fathers have set.