Great job getting all of your seeds to sprout outside like that! You must have given them good attention.
Bugs are hard to avoid unless you start your stuff in pots and then put them in the ground after they have a few sets of leaves<<even then one slug can screw the whole deal up.
If ALL the leaves are gone, it will probably grow back if the top was not taken; otherwise, it may be a sad thing. No harm in giving her the benefit of the doubt though since you will be there watering the others every day anyway. Just keep treating her as you treat the others and maybe she will recover.
Slugs can be kept away by using a cup like you were thinking but the air sounds too warm for that. Insulation like a cup raises the temperature a great deal. You can try other barriers to keep your plants safe though. First, scratch around in the dirt next to your plants to make sure that the bugs are not hiding there>>we do not want to lock those bugs in with your plants. Next, make some barriers. Copper is a favorite. If you happen to have any electrical wiring around and it is copper colored inside, you can strip off the outer plastic coating and make circles out of the wire to put around your plants. Do not put the wire on your plants, just around. Salt can work for slugs but do not use a lot, maybe a line .5cm wide and put the line at least 6cm away from the stalk of your plants. Ashes can work too. Put the ashes even farther away from you plants, maybe 10-12cm and make the lines of ash about 1cm wide. If any of the ash is still there by the time your plants grow 10cm tall, then take it off of the soil and pour a new line further out>>>the line should be past the root zone. I get my ash from my woodstove but if you make a fire to get yours MAKE SURE IT IS SAFE and doesn't set your place on fire..setting your place on fire is very easy to do, I have done it; wind is a big danger. and dry grass, pine needles and leaves. Have something with you like a shovel so you can put dirt on fires. or whatever, maybe a fire extinguisher.
Since they are new born, you may want to give them half strength fertilizer in their water. And I think it is a good idea to start that right away. After a week or ten days, then you can switch to full strength fertilizer and they will probably do well if you dug the holes nice and wide and deep before you planted them. if you dug no holes, you may want to carefully dig under your plants and move them for a moment while you dig them nice holes but be careful of exposing their roots to the sun and when you put them back in their holes they will need a very good watering. Holes need good compost or black dirt or leaves or grass or old manure; put something in there to make the dirt very nice for your plants. Sometimes you can find compost under trees where the leaves have decayed; you can gather the top 2.5cm-5cm of dirt and use a bunch of that in your holes. it is good to have at least 1/4 of the stuff in your hole be compost and such.