Those bolts are not "self locking" like QR system, but I've never had a properly torqued one come loose.
Is it a real risk leaving those hooked washers out?

There would be no reason to do so.

When installed properly the hook sits in a little hole on the fork and does nothing. With the nut tight, the hook has zero function. The hook only has a function if the nut comes a little loose.

The hook part doesn't interfere with the washer function of the part at all. It's just a circle with a hole in the middle like any washer.

If you replaced it with a standard non hooked washer, it would do the exact same thing, without the safety feature of having the hook if the nut did happen to come loose.

The hooked washer will work properly with a nut on the axle, a wing nut on the axle, or if you're just using The Force to hold the whole thing together with your mind.

Once you figure out what diameter/threading your axles have, you could go for some old school wingnuts. They used to be fairly common decades ago and have big enough 'wings' to get your wheels secured tightly enough by hand. Just search eBay for 'bicycle axle wingnut'.