What a lovely skirt! Gorgeous colour.
You don't need to join the elastic in a loop before inserting it into the band, if you are willing to accept a cheat fix
Here's a cheat you might like to try. Sorry the post is so long - it takes far longer to type out than to do! It assumes your band is open internally all the way along, that it closes with a buttonhole rather than a button loop, and that the buttonhole is already made.
You need to be able to open the end of the band at the button end, plus open part of the band at the buttonhole end. You'll also need dental floss or other very strong smooth thread; and a strong tapestry or other blunt needle. If you want to, before you start you can overlock or straight stitch or otherwise finish the cut ends of your elastic, but given how little strain there will be on the elastic in this skirt - it almost fits now so very little pulling on the elastic - I don't think I'd bother.
First, prepare the ends of your band, and the ends of your elastic.
For the band: Remove the button and completely open the vertical edge of the band at that end. Open the vertical edge of the other (buttonhole) end of the band as much as you reasonably can, remembering that what you open will need to be neatly closed.
For the front end of your elastic: You will need a *very* long length of dental floss; at least twice as long as your band, plus at least 40-50cm. Using your tapestry needle, pass the floss into the elastic at one end, taking a generous (2-3cm) bite of the elastic. Leaving the floss in the needle, tie the two ends of the floss off around the elastic end, using the strongest knot you know. The floss should now form a very long loop, running through the eye of the tapestry needle and through the elastic.
For the tail end of the elastic: Pass a loose loop of dental floss through one end of the elastic, taking a very generous bite (maybe 5-6cm) of the elastic. Knot the two ends of the floss around the elastic. You should now have a loop of floss, which will be your handle when finishing off.
Using the eye of the tapestry needle as a threader, thread the floss into the band from the button end of your band towards the buttonhole end, being very careful not to allow the eye to pass through any loose threads etc, as these will stop the elastic passing in the next step. Once the needle eye is at the buttonhole end, pass it out the pre-opened section of the band. The floss will emerge with the needle. Find the knot in the floss, and make sure it's outside the band. At this point, you should see the needle with the knot in the floss completely outside the band at the half-opened buttonhole end, while the elastic should be outside the band at the fully opened button end. The elastic and the needle + knot are joined by the loop of floss you've just threaded through.
Holding both thicknesses of floss and using it as a "puller" from the buttonhole end, start working the elastic into the band from the button end. Work the elastic past all the obstructions which will be put there solely to frustrate you (eg the side seams). When the elastic reaches the inner end of the buttonhole, pin it firmly in place. I use a safety pin for this; after all that work I don't want it moving till I've sewn the sucker down. Don't sew it down yet. Leave the long loop of floss hanging out for now, and move to the other end of the band.
The "handle" you put at the tail end before you started should still be hanging out the button end of the band, waiting for you. Make sure the knot in the floss is outside the band. Try the skirt on and adjust the length of elastic if needed. You can use the handle as a "puller" until you get the elastic anchored at this end. Once you're happy with the length of the elastic, safety pin this end in place. Leave the loop of floss hanging for now.
Check to make sure you're happy with the placement of the elastic inside the band (not too close to one end or the other; not twisted etc). Then stitch the ends of the elastic down. I simply stitch it in place with a vertical line of machine straight stitching probably 1cm from each end of the elastic. Close both ends of the band. Replace the button. Try the skirt on to be double sure that you're happy. Only then and as the final step, cut both loops of floss and remove from the band (it should simply pull out).