You have it right...1:1 in the spring to get things going. You can stop feeding when the girls start bringing nectar in. I don't know your area but check with local beekeepers for when you get your first good flow. Last year, my bees stopped taking syrup as soon as the weather got warm. George is right. Bees should be collecting nectar and making honey. Syrup is what you to help nature along!
If your bees are taking a lot of syrup in, it won't last long enough to go bad. Last fall, I supplemented 2:1 and they'd clean out 2 gallons in a week. Chef Isaac gives good advice regarding adding a little vinegar. That'll slow mold down. Just pop the top once in a while on your top feeder and take a peek. So, you can leave it on somewhere between days and weeks.
Zip lock bags, laid across the top bars with small slits on top for the bees to sip from will work fine. Don't overfill. You might need to use a shim to give yourself some space for the bag between the super and the cover. Again, don't overfill. A shim is a short "riser" that creates a little extra space. If it were me, I'd use the feeder I have. If you're worrying about the syrup going bad, don't put so much in. If they use it...add more. If they don't, you're not wasting much.
Does this help?