Go with the PVA, seal with a very dilute mix and just before applying the plaster apply a second adhesive coat approx. 50:50. I would then fill with bonding coat 11mm at a time. Strike off any excess with a straight edge or Derby filling any depressions. Scarify the finished surface once set and apply finish whilst base coat has plenty of moisture. If you have to apply finish at a later date then seal the base coat surface again with dilute PVA.

One of those holes looks like the plaster on one edge is coming away from the wall, you need to enlarge the hole until you reach sound plaster. Using a hammer and chisel may cause more plaster to loosen and you end up replastering more than you wish. A gentler way of removing the plaster is to tap it to establish where it is sound and use a carbide blade in a multitool to cut an edge then use a decent scraper to prise away the plaster to your ground edge.

Richard