I second the opinions about putting (at least) the 3 youngest back RFing... and reharnessing the 4yos.
I, too, am a daycare provider, and had a bit of a shock when I found this board last year and realized how unsafely I'd been transporting my dck. (The irony of the situation is that I had been transporting many of them *more* safely than they rode in their own vehicles... I always RF'd to 1/20, FF'd to 40lbs and boostered to 8 & 80.) The idea of having to buy a half-dozen new seats was really daunting... as I read this thread I thought it might help to make some suggestions to you with that in mind.
(As I say all of this please realize that I'm not a tech... it's just my opinions based on what I've read here, and based on my experiences.)
The oldest few -- your child 9, daycare child 8, yc7, and *maybe* dck6 are fine in boosters, preferably highback. If your van is like mine (and I believe most 15 passengers are,) the safety ratings are not great and since I can't get a new vehicle, I try to afford the kids every protection I can.
Both 4yos should be harnessed, if not RFing. I love my Graco Nautilus, but when I first started replacing seats, it seemed very expensive. I purchased two Apexes (which have their own caveats,) but since then a few more higher weight harnessing seats have come on the market. You can look at the Evenflo Generations or Maestro for more budget-friendly options.
As a general strategy, I have purchased seats that are more expensive and put them into use at the top of their "use range," knowing I could pass them down, while using what I had for the kids who more easily fit into those categories. Rather, in plain English... I bought a MyRide for a 38lb 2 year old, knowing that he wouldn't have much time in it, but also knowing that I could pass it down to the 4 month old I had. In the meantime, I put the baby into a Scenera. Likewise, I bought the Nautilus for a tall then-7yo, knowing that she'd only last the summer in it, but also knowing that I could pass it down to a 4yo, who I put in an Apex for a while. I used what I had as much as possible, replaced the most critical seats first, and went from there. (I think it took me about six weeks to reseat everyone, and a few have gotten new seats yet again.)
So to apply that to your situation... you have an easy fix with the 18mo, just turn the Cosco Alpha around backwards. No new seat needed, five times safer child. (That seat is only 20% as effective installed forward facing!) I know you'll have to replace it within the year, but it will at least get you started while you focus on seats for the other kids.
For your 2yo, you might want to look at a MyRide or TrueFit... depending on whether your child is shorter and more "solid," or thinner and taller. The MR rearfaces to 40lbs, but has a shorter shell than the TF. The TF, on the other hand, has the taller shell but only goes to 35lbs RFing. (It's hard to know if "2" is "just had a birthday," or "is almost 3," so it's hard to know how the child is trending.)
If you're looking for an inexpensive seat for the baby, you could pick up a Cosco Scenera, or a Safety 1st Avenue. Or, for that matter, the babe could use the second Alpha Omega, until it expires.
Other frequently recommended convertible seats are the Complete Air (very tall shell, rearfaces to 40lbs,) the Evenflo Triumph Advance (rearfaces to 35lbs, but has super easily adjusted harness, which makes it great for sharing between kids!) and the Radian, which as noted above is the only seat that rearfaces to 45lbs (if you're interested in re-RFing your 4yo.)
As for why we're all going on about rearfacing... as mentioned, it keeps your kids five times safer, at least through age 2, and a recent British study indicated it's beneficial through age 4. And that's only what's been studied... although it's always safer to rearface (for everyone, including adults!) the safety benefit does decline at some point, I believe between ages 4-6. The danger, particularly to the under-4 set, is that a child's bones don't ossify until around age 4 and this means their neck can stretch 1.5-2" (I think that's the correct figure) in an accident, but the spinal cord can stretch only around 1/2" in an accident. Check out Joel's Journey on youtube to find a story of a grandfather whose grandson has gone through just that. His Grandpa is now a huge "extended rearfacing" advocate.
Also... a reminder to tether your forward facing seats to reduce head exertion. I didn't think my van had tethers, then got told otherwise by the lovely ladies here, lol.
Sorry... I know that's really long but I felt really compelled to say a few things, I think because I identify with needing to get car seats by the batch, lol. And of course my needs change as my daycare kiddos change...
One final thought as I reread your posts -- if the manual does not expressly prohibit 3 carseats per row, a Radian might be your best bet for those middle seatbelts, since it's the narrowest seat on the market. But, it sounds like you have more seats than kids, so you might not need to fit 3 in a row. If you definitely do, then some of my above suggestions might not be the best.
Chickabuddy... I think there was a 50lb Eddie Bauer that I read of somewhere, when checking a harness limit on a seat one of my dc families had. But I've never seen a 60lb either.