Most of the IP’s I have done used affinity-purified antibodies from immune sera. There wouldn’t be much point to using pre-immune in that case, although you could use an equivalent amount of non- or pre-immune IgG (purified by Protein A/G agarose, for example).
I would just do the mock-IP in parallel, rather than doing the pre sorption step (although that works, too). So you’d make one extract (or two w/ one being the mutant; ideally made at the same time and never frozen), having the same concentration. Pull off the total samples from each for the western blot, then add the IgG coupled to the beads (control or affinity-purified), nutate for ~2 hours to overnight at 4°C, pull off the flow-through (take a sample of that), then wash the beads a few times like you did, and then elute in non-reducing sample buffer. Decant the elute, reduce with DTT or B-ME, and reboil. Then run the gel with all the samples, total (both lysates), flow through (both lysates, both IP’s), and IP’s (both lysates, both IPs). It’s a lot, but all the information will be there in that one experiment.
As for making the affinity-purified antibodies, that’s kind of up to you. I like injecting a His-tagged protein into the bunny, then making an affinity column with some other tagged version, like GST- or MBP-. This way you’re not just isolating anti-GST antibodies but only the shared thing that was in the two protein preps. I then couple the protein to a resin. Affigel 10 or 15 works well from BioRad if your protein doesn’t have cysteines, but if so, like a GST-fusion, you can use SulfoLink from Pierce. Couple according to the instructions, quench, and mock elute, typically 100 mM glycine, pH 2.5, and wash back into PBS, add azide and store until you’re ready for use. I usually add ~10 mg’s a protein to 1 ml of beads to make the column, store in PBS with some sodium azide, and you can reuse the column for years. Add your antisera (I usually add 1/10 volume of 10x PBS) + NaAzide, and mix for ~1 hour or so, wash with PBS, and then elute with 100 mM glycine, pH 2.5, as above. I take 0.5 ml fractions into tubes w/ 50ul of 1 M TrisCl pH 8.5 or so, to normalize the acidity. I usually take 5-10 fractions, find the peak by Bradford assay, and pool the peak. I usually cross link 100ug of antibody per ml of protein A beads according to the instructions in the Pierece kit, and I use 20ul of beads per IP. But your mileage will vary.