Those are excellent questions...
Your first question is actually trickier than it sounds.
Because of how it works, the error correction on CDs is able to repair single relatively serious flaws, but can be "overloaded" by a large number of smaller flaws.
This suggests that it's a good idea to avoid adding any more smaller scratches or flaws than absolutely necessary.
It's also generally recommended, at least for a "final polish", to wipe radially (from the center out or from the outside in).
Next best is small swirls or ellipses.
Worst is to go around the disc parallel to the tracks.
However, the data on a CD is actually stored on a layer below the surface, and read by a LASER that is focused below the surface, and operates in a certain color range.
Because of this, the read mechanism is able to ignore many small surface flaws (again, depending on exactly what sort of flaws they are).
Unfortunately, this means that the answer depends on nit-picky details like the exact size and depth of the scratches you're adding, and even the direction they're running in.
All polishes work by making ever-finer scratches... the catch here is that what looks clearer to the human eye may not be exactly the same as what looks clearer to the read LASER.
(So those "dull elliptical swirls" may look perfectly clear to a red LASER focused below the surface... or they may not.)
In your case, though, your second question "informs" the answer that will apply to the first one.
If you were playing your CDs on an audio player there would be cause to wonder if significant amounts of "error concealment" (interpolation) was occurring.
However, because computer drives generally do little or no interpolation, as long as you're getting a "solid read", that probably means there are no uncorrectable errors, so there would be no point in going further.
A good compromise, if you want to be as sure as you possibly can be, would be this...
RIP the CD once and save the results...
Then give it one last "final polish" and RIP it again...
If you compare the two resulting digital files, and they are identical, then your "final polish" didn't make any difference...
(A lot of file management programs can generate checksums... to compare two files, simply create a checksum of the first ripped file, then "test" the second one against that checksum.)
Since minor scratches usually result in "soft errors" that change each time you attempt to read or polish them, if the results of two subsequent rips are the same, then the data you've read is probably perfect.
This is sort of a variation on the idea, which some software still offers, of reading the disc multiple times and assuming that, if it reads exactly the same multiple times, then it's probably good.
It's also usually a good idea to open the drive, then re-close it, between reads.
This rules out the possibility that a slight shift in the position of the disc in the drive may affect the results. (
Many programs do this automatically when they "verify" discs.)
I haven't used EAC in a long time, but older versions used to offer a "secure read" option, which was independent from AccurateRIP, and which involved reading each sector multiple times (I think at different speeds).
If multiple reads produced the exact same data then it would assume the read was "good".
And, if one or more of the attempts produced identical data, but one or more did not, it would assume the version that was identical across multiple attempts was the correct one.
(I sort of remember some versions falling back to doing this if they failed to find the disc you were doing in the AccurateRIP database.)
Either way, once you've got a good read using EAC, using its secure mode, there's probably no reason to keep polishing.
Three other thoughts here....
First, there are several products on the market for "making foggy headlights clear again", and for "repairing scratched sunglasses and safety goggles"...
Rather than polishing out scratches, many of these work by filling in surface scratches with plastic, or with some sort of wax, which matches the optical properties of the original plastic.
(Most of them you apply, let dry, then "buff off".)
I tried this a long time ago with a commercial product for repairing the lenses on safety glasses - and it worked quite well (that particular product is no longer available).
I suspect that some of the newer products may work even better (especially since CDs are made of a type of polycarbonate plastic rather similar to modern headlight lenses).
I also suspect that some products designed to "remove scratches from the clear coat on cars" or "actually fill in the scratches" may work very well - but I haven't tried any of them.
Second, there are a few commercial music CDs that incorporate various types of copy protection that works by deliberately storing some "bad data" on the disc.
The idea is that they've incorporated "special errors" which won't produce audible flaws, or are "successfully" repaired by interpolation, but will cause the disc to be treated as a bad disc when you attempt to copy it.
Some of these discs have trouble playing on some ordinary players, some won't play at all on a computer, and some end up with multiple loud audible ticks when you try to RIP them.
There are several of these systems, none of which was very widely used, and most of which were used by only one or two manufacturers on certain discs, but the discs still turn up from time to time.
So, if you find a disc that "just won't RIP but looks OK", before you go nuts, try doing a Google search on that particular title or ASIN.
(This has been annoying enough people long enough that you will probably find it mentioned somewhere in reference to a particular disc if you look - and you can find lists of discs that use each.)
(Also, some programs may be more or less successful with RIPping some of these, and many of them were eventually reissued or replaced with new versions without the protection scheme.)
Finally.....
On audio CDs, the recorded layer is sandwiched between two sheets of clear plastic which are actually quite tough...
On CD-Rs, the recorded layer is in the top (label) side of a single clear sheet of plastic, with only a relatively thin layer of lacquer or plastic coating over it.
(That's why it's dangerous to write on the label side of CD-Rs or to pull adhesive labels off of them.)
The plastic on the front surface is actually quite thick.
I actually tried to repair a few very badly scratched discs once using a buffing wheel attachment on a grinding machine.
I was successful with one, but the other one overheated, the adhesive softened, and the disc actually fell apart.
Thanks, this info can be useful to me - a family member's in-car CD player gouged quite a large number of my CDs over many months, so I'm reluctantly in the CD surface repair business.
This fact leads me to wonder about my polishing technique. When using a coarse grained polish to remediate a deep scratch, the CD's "mirror" shine is lost and replaced with a dull, mat, ugly surface with eliptical swirls. I tend to feel obligated to keep on polishing with a finer grained polish to recreate a more (not perfect) shiny surface, but often wonder whether that may be just a waste of time. I just want to rip it with EAC and store it away.
For a CD with no Accurate Rip data available, should you stop polishing the moment your drive reads the CD without skipping/stopping, or is there actual benefit in polishing the surface further ?