I guess my question here is
would the scale be on the roots and/or in the soil?
I removed the badly infected growth and scraped off all the visible scales.
This has happened before - after bringing the plants indoors for the winter. I have lost some plants due to recurring outbreaks of the scale.
Have had no luck with systemic stuff.
Not much luck with soapy water....

Try Neem oil, Soapy water that you washed out a creasy pan, try 50-50% alcohol.They may be already dead and should be after what you have used. Use a soft tooth brush to clear them away, they will not just fall off by themselves.

Bal, I cant testify to the Neem OIl...that is suggested to treat black spot and mildew on roses; you might try it.

But the oil you are familiar with when applying to your outdoor deciduous trees, shrubs and roses..."dormant oil" is another suggestion. Do read the label.
Also suggested is the "lime sulfur"...the other half of the dormant oil application. Smelly stuff this...so open a window and do read the label.
No success with soap.
Perhaps you didn't do the routine as best prescribed.
A 40 to 1 solution dishsoap and water, spray from top to bottom, bottom to top, leave on for 10 - 15 minutes...then rinse well in clear water. To avoid overwatering the surface soil, you might tip the plant to a 45 degree angle or cover with plastic when applying

Anytime you have a problem with a pest, it is usually prescribed to treat the plant as many times as needed to break the cycle of the pest's ability to produce larvae.
In a 20 day period, treat the plant 3 times.

Scale insect does not as far as I know, ever go to the roots, they feed on leaves and stem, sucking the juices there.
They leave behind their secretion called "honeydew"...a shiny appearance on the leaves can be seen.

I had some epiÂs with scale & after an unconvential home treatment got rid of them. The oil is usually suggested to smother the scale. It is probably armored scale, & there are about 200 babies underneath each scale. The babies are crawlers & pretty difficult to see. I think you can see them with tape or yellow tape, but my hard drive is down & I lost my notes. Scale is not supposed to infest the soil because they crawl away, but one of the things I did was get rid of the soil. When I looked at the plant roots, I also found some armored scale at the base of the roots. If you are missing these, you have not adequately treated the plant. You have to be very meticulous in getting rid of this bug. It is time consuming, but can be done. I treated 50 at once  now before I get in any new addition, I immediately inspect & treat if necessary.
It is important to recognize scale on these plants as some major vendors sell cuttings with it. It is worth the cutting, if it is a rare cutting. Scale can also be transferred from plant to plant by wind or by fabric - such as clothing touching it.