Those look like plaster cracks, not paint cracks (especially the last one). Latex goes over oil just fine with proper prep. The first step is a thorough cleaning. Washing before painting walls seems to have become to 'old fashioned.' It actually works. Even with primers designed to stick better.

According to the spec sheet: For optimal performance, this primer must be topcoated with a latex, alkyd/oil, water based epoxy, or solvent based epoxy coating on architectural applica- tions. For exterior exposure, this primer must be topcoated within 14 days with archi- tectural latex or oil finishes. For better performance when priming an entire house, use Exterior Latex or Oil- Based Primers. If you like the look, keep it protected by applying a coat of flat acrylic polyurethane. Here is a link that might be useful: Primer

Thank you so much for telling me about Trim Magic. It may do the trick. When we bought our house the trim that we removed had been painted over the shellac without being primed or properly prepared. The paint was split with the old shellac finish showing through all over the house. The wood was poplar. We decided that it just was not worth the effort to strip all that poplar. My husband milled new trim in the original style but in white oak. I did strip all 54 of the turned balusters and the rest of the staircase. It took a heck of a long time to get it all clean enough to be refinished. There were some broken parts where my husband made repairs with new poplar. We found a great guy who has done finishing work in some important old New England homes. He did a wonderful job of getting all of the sections to match. He had to do it in a walnut color to achieve a nice allover look. He used a Zar gel stain. The stairway stands as a sort of antique in the middle of the house. He really made it glow. Part of my concern about sanding or scraping the lead paint is that we have two small terriers. I would not want to expose them to the dust and chips. I am very grateful to all of you for your advice. Michele

You know this is just one of those things to write on a can of primer so that when their product fails, they can blame you for not applying the topcoats fast enough. I have never in my 30 years of painting had an issue with paint or primer failing because I waited too long to put the topcoat on. Sure, it is best to always follow manufacturer recommendations, but this is a perfect example of what do you do when the manufacturer recommends nothing? It's a clear sign that this probably just isn't all that critical of an issue. What I always do though if I can't get back to apply the topcoats in a timely manner (within two weeks or so) I will sand the primer just to help that paint stick better in case there is anything there that has collected on the primer that would impede that adhesion. Enjoy your Christmas and don't stress out too much about this.