Yup, bacterial wilt. Use of sevin or rotenone/pyrethrin just regularly on the young plants MAY help. You need to stop using it when blooms open, so you don't kill off the bees and other pollinators. Also, there is an agricultural antiobiotic called Agro-strep (a streptomyacin) which works on many bacterial diseases, if you can find it. Terramyacin powder is sold to control bee and poultry diseases, and can also be applied as a spray on plants. But, this also needs to be done as a preventative measure, or at the very first sign of any disease in the patch, hoping to save the plants not yet showing symptoms.

It sounds like the base is rotten and not salvageable. Most likely the once-a-week watering you started with was too much, if it's planted in a regular peat based potting mix, and even the initial signs that you saw were those of overwatering rather than under watering (annoyingly, they're the same signs). You should be able to cut off the last pad if it's still healthy and root it. Make sure there's absolutely no sign of rot in the section you're saving, cut away anything that is suspect. Set the cutting aside for several days to let the cut end dry out and callous over. You should then be able to plant it into a well- draining mix and allow it to root. Look up gritty mix, or use straight pumice, or a bagged succulent soil that is heavily amended (50/50) with perlite or pumice. Don't water it at all for at least a couple of weeks. Once the cutting feels really stable in the mix, that's an indication that roots have formed, and you can start watering. Let the soil dry between waterings, though a gritty type mix will be much more forgiving when it comes to watering frequency. When you see new top growth you can add a little fertilizer.

It needs more sunlight. Also, I recommend a 50/50 Cactus/Succulent soil and perlite so it can have good drainage, it may rot without it. It looks way overpotted as well and I am wondering if that pot has a drainage hole

I agree with stupidlazydog on the light issue. I only have 2 Euphorbias, but one is a beautiful crested euphorbia lacteals. I live in FL and both are outside on my lanai. They get 3-4 hours of direct FL sun in the summer, and even more in the winter. And they seem to handle it well. Agree too with the suggestion to stake the plant and acclimate it to stronger outdoor sun, if you move him.