Good questions!
In general (from my experience), it's a good idea to establish a solid "base" of about 10 planets or so (looks like you're doing that), then go straight for the AI homeworlds.
On a 40 planet map, you may feel as though you are "missing out" on something by avoiding planets that are "out of your way", but when you play an 80+ planet map, you will get used to having to avoid a lot of potential technologies and incentives that go with increasing your AIP a ridiculous amount.
In AI War, you are constantly fighting against the clock. The longer the game goes, the more powerful the AI gets, and their power is exacerbated even more by high AIP.
The bottom right cluster of planets may indeed have some nice fabricators and incentives for you, but to me it seems too far out of the way, and would be too time consuming a process for what it's worth (not mentioning the additional AIP you would accrue). Some others might disagree with me on this, but my experience with the game dictates that you shouldn't take unnecessary risks to help you win the game, and Core Fabricators have always been a luxury for me, never a necessity (as in, you can easily win the game without them).
If I were you, I would go straight for that P1 Planet at the far right/mid-top part of the map, and then from there make a "beeline" straight for the enemy Homeworld(s). Remember that it is important to unlock MKIII scouts, so that you can produce an MKIV scout with your MKIV Factory. MKIV scouts
can not be detected, and will help you reveal the rest of the map, so you know exactly where to go.
To answer your question, AI Homeworlds can not be "cloaked", so by process of elimination, you know that are on the left side of the map, fairly close to each other.
For a more advanced strategy/technique guide, you can check this link(
http://www.arcengames.com/forums/index.php/topic,7686.msg64478.html#msg64478), where I gave a pretty detailed synopsis of how I play.
edit: Also, please note that if you are running the latest version of the game (4.41), there is a bug with Counter-Attack Posts that will basically cause you to lose the game if you kill one. If you are running v4.41, DO NOT destroy a Counter-Attack Post, at any cost.