You can use them. Not all of them may germinate so you may have to plant a few more seeds than normal to get the number of plants you want. My suggestion would be to plant whatever you have left and then maybe make a trade via Freecycle or Craigslist for whatever you don't want with someone else. I know my local Freecycle has a gardening board and members swap extra seedlings for other seedlings to make their gardens more interesting or to get a chance to try something they normally wouldn't have spent the money on.

Not enough information - Seeds for which perennials. Some will store better and not lose their viability as quickly as others. Do they have a dormancy that needs to be broken, by a cold moist chill, or would they germinate if you direct sowed. Freezer really isn't helpful, although if a cold moist chill is required before they will germinate, your refrigerator (not freezer) will work. But, if they need a few or several weeks of moist cold before being brought back to warmer temps to germinate, you could be looking at late summer or early Fall for seedlings. Then what, where will you keep tender seedlings that haven't had time to grow any kind of root system over winter in a Z5... So again, which perennials, you could ask here and one of us may know, or you can look them up on Clotheirs database to see what temps the seeds may need to germinate and an approximate time frame Here is a link that might be useful: Clothiers

They come from a good keeping variety; so they should keep as well if not better than the bigger onions you set for seed. Onions are a perennial and so I am not surprised that they multiply. I wish I knew whether these will grow back into larger bulbs like the smaller bulbs in the multipliers do or whether they will simply bolt and set seed or even if maybe they will simply stay as multipliers. If properly cured they should keep into next spring and allow you to plant them back and find out. Let me add a peripheral note: I keep the smaller Copra bulbs I get from spring plants and plant them back the next spring, more less as oversized onion sets. They size up pretty nicely the second year, and I have yet to have any of those bolt. So I can't say about the seeding ones. I haven't had any seed out yet.

I am in z5 in central CT. The ones that return year after year were discovered by accident. I forgot to dig them up once about 10 years ago. When they returned the following summer, I stopped digging them up all together. Half of them are located immediately next to a concrete foundation porch with a south-east exposure and the second half are located about 3' away. They may have an unfair advantage because of location, location, location. I think they are tiring out though, I do not see as many this year. They could also have been buried deeper than usual. They could have been a special breed. I have no idea. I set in another batch in the fall in a far distant section of the property and I see a few poking their first leaves. We did have an extra ordinary mild winter this year so that doesn't prove that they will survive the winter and come up next fall. I take pics of everything. I am surprised I don't see one of these.