1) Igoumenitsa-Corfu: Here's the current schedule of the main line (
Kerkyra Lines)and as you can imagine, this is valid for the winter. The summer schedule is not yet out, it should be there in a couple of months and it will include a few more departures (also during the Easter period). There may be additional ferries from other companies but, to be honest, I did not feel like searching a lot. As for when to book, other than during the Easter period when the island is literally invaded by (Greek) tourists and the time around August 15 ± a week when pre-booking is definitely recommended, you actually don't have to book ahead of time, you walk up to the harbour agency and buy tickets for the next one leaving for Corfu.
2) Corfu-Patra: There are two possibilities for that. The first one is a small and slow one that leaves two (or maybe three) times a week. It takes alomst two days to get to Patras as it stops on every single Ionian island. Unfortunatey I don'y have details for that and I don't know whether they'll have it again this summer. The second possibility is to use one of the main Italy-Greece ferry companies that serve Corfu such as ANEK lines. Unfortunately, I don't really know whether these companies still take passengers for the domestic leg, I think that they stopped that as it may mess up their schedule and there were always very few people using it.
Finally, considering the new highway to/from Igoumeitsa to Patra, or the use of the "old" but nice road from Igoumenitsa to Preveza and then the highway, this may also be a possibility for you.
PS You shoule consider asking a Mod to open a new thread for your last post and my reply. Actually neither Corfu not Igoumenitsa count as Thessaloniki "surroundings" and any info, including future one on the particular matter may disappear for potentially interrested FTers.