All I can do is give advice about what works best for me, and I can't stress enough that "your mileage may vary".
My personal preference is to keep the lid on loosely, slightly off so that two adjacent corners are cracked no more than a half an inch. I take the lid off every two days, use it as a fan to circulate some fresh air into it (three or four back-and-forth waves over the opening), then check my cups for water content. If everything is OK, I put the lid on, partially cocked again. I repeat this throughout rooting. I keep my bins in a room with lots of windows, but NEVER in direct sunlight. I actually keep them against a southern wall about 2' under a window sill, so the light can never contact the bin.
I know some people like to use saran-wrap tents over their trees or their bins. I'm not knocking it, so don't get me wrong, but that is a hell of a lot of extra work for something that is basically a weed (whether we like to admit it or not, ficus is weedy) and will grow with minimal work on your part.
The #1 mistake I made when rooting was not properly watering for the potting mix I was using (chunkier mixes have diff't water requirements than fine mixes), and I honestly believe this is the most common mistake everyone makes with any plants, not just figs. The #2 most common mistake being "caring too much"; you can easily love a plant to death, this is especially true of starting figs.
You have three forms of light: direct (or "unfiltered"), filtered, indirect. Your figs should not be getting direct/unfiltered sunlight at this stage (the one fig is getting close, but still needs time). When it is time for some direct sunlight, you either need to filter it (frosted window or light sheer curtain) or limit the amount of direct light. Mine usually take about 2-3 months after the first leaves are forming before I get to a point where I'm even thinking about introducing filtered light, much less direct sunlight. You should have at least 2-3 fully formed leaves before thinking about moving to filtered sunlight. Direct sunlight will absolutely burn leaves that were not grown in direct sunlight, and I can show you what it looks like when this happens!
As for whether it's getting enough light ... Think about it ... I've had cuttings start leafing out in bags in rooms with no light. Others here at the forum have had plants come out of dormancy under their houses and grow several inches and leaf out with zero light. So, really, putting a lid on top of your bin is not going to hurt these guys. They're not manufacturing a ton of chlorophyl at this stage (not enough leaves), so why on earth would they need direct sunlight?
Remember - these things are cuttings, they started as pieces of wood - for the first two months after roots show up, they will probably have less than 10" of hair-thin roots and maybe 1-2 square inches of leaf surface total. I cannot express to you enough how minimal their needs are to stay alive when they have so few roots or leaves. All they want you to do is provide a pinch of water to stay hydrated, and they'll do the rest for the first 6 months or more. If you need any more evidence of how little their needs are, there are some interesting pictures all over the internet and this forum of figs growing out of gigantic boulders and mountains of solid rock with absolutely zero soil/nutrients.
As for your medium choice (potting mix/soil choice) I don't have a problem with Miracle Gro, personally, although I try to encourage people to stay away from 1) anything with fertilizers, these are usually little round bead looking things, and 2) avoid anything with the word "soil" in it - "potting soil" and "potting mix" are NOT the same thing.
I would also share this ... as you go along, I would play with mixing your potting soil with perlite or some other light, big, chunky stuff while trying to root from cuttings. If I "conveniently" forget my Turface/Sphagnum trials (which was a waste, personally), I actually started just like you, with a full-on potting mix and no chunky additives. I kept having problems with overwatering. Within two batches of cuttings, I had moved to a 50/50 mix of plain old chunky perlite and potting mix, which resolved 90% of my watering problems because water would drain right through and not sit directly on the roots. Now, at this point, I'm doing something like 80/20 (80% chunky perlite, 20% potting mix). Some guys, like Jon at Encanto Farms, use nothing but perlite, maybe adding a sprinkle of compost or straight perlite with "compost tea" for watering.
Again - I will remind you - the purpose of potting mix is to provide a moist, nutrient-rich environment for a full-grown plant to thrive (my opinion!). These cuttings aren't full-grown plants. They get the eenergy to root and grow leaves from inside the wood. They don't need a lot of soil. Even in the wild, full-sized trees can grow on top of rocks with no soil or nutrients. For these reasons, I do not think it is necessary to provide soil-heavy blend, a fig cutting will root happily in a cup full of rocks for months, if not years. Don't let the idea that "more nutrients = better" get stuck in your head. Some plants just do not like having an abundance of nutrients.
The best thing I personally think you can do for your figs is to provide a big, loose, chunky mix. This will provide air pockets (allowing humidity) for the roots and prevent you from drowning the roots with water. I only keep the15-20% of soil so the trees have "some" nutrients when they hit the 4-5 month mark and I can usually keep them cupped up longer.
For some reference: An experiment of leaving your new rootings in cups versus transferring to pots ... http://figs4funforum.websitetoolbox.com/post?id=4912281
Problems you might encounter while transferring from cups to pots, and some advice from other members (see James' comments, especially, for some sage advice) ... http://figs4funforum.websitetoolbox.com/post?id=4602240
I also hope that some other folks will come in here and share their experiences, or at least confirm that they generally agree with what I'm saying - we all have our own methods and madness. I always stress - never, ever, ever take anything someone here tells you as the "definitive word", because rooting is still mostly voodoo sprinkled with a little luck and some basic plant growing fundamentals.