Roller marks can't be simply washed away, and dependent upon which coat the marks developed on, sanding isn't a reliable fix. In any case, the surface finish is a primary source of brightness, so sanding will mute that somewhat. Really, it's almost impossible to judge what might be a viable fix, as being remotely removed from the location doesn't allow for a accurate assesment.
$300.00 for a "Pro" to roll a screen, eh? Well if that isn't a testimonial for spending even $125.00 for a good Electric HVLP Gun and making an attempt at spraying one's self.....
It seems no matter how much the ease and affordability of doing so is elaborated upon, some folks just run screaming from attempting such themselves.
The RS-MM LL mix as listed has a paint component / water amount that is ideally suited for spraying. If rolling is attempted, the a a Pro should know that it must be done using a very low nap roller, and applied very wet, with very little back rolling...much like applying straight Polyurethane with a roller. One must always leave a wet edge at a row and then overlap that edge with wet paint....never roll out the edge. He also should have judged the viscosity of the mix as he was mixing to determine how much...or how little water he should add dependent upon his own rollers and technique.
It's not very easy or even fair to overly critique your Painter, but you did pay him pretty good to paint a single wall, and I'm sure he knew going in it was needed for a meticulous job to be done so something extra needs to be expected of him...it's not all dependent upon the paint.
That the painter needed to apply 5 coats tells me he applied them far too thinly, and wound up over-working at least a few areas with his roller. The very fact that a Pro still would still wind up with a few roller marks only cements the proposition that most Painters are not used to "Painting Screens" and employing the needed methods....they paint Walls where intense light is seldom projected on, and use paint that doesn't have the properties within to enhance contrast to a point where normal methods might not manifest such issues.
If the paint is formulated to be thicker, one still must use a low Nap roller and work wet edges with a minimum of passes. It's all about rolling technique, and frankly, trying to teach the minions out there how to roll contrast enhancing paints has always been fraught with frustration. Spraying was always the preferred method, but for years it was far too expensive. That changed back in 2004 when electric HVLP Guns came into the fore, and that change heralded in and made practical the use of paint solutions that were simply beyond the use of those who only used rollers. Even so, some have steadfastly resisted spraying as even being necessary, instead blaming the paint itself, or even the need for such paint. Never minding the fact that it is the paint that makes such results possible...along with correct application.
Well, all the above fussin' doesn't do you much good, Jack...I know. I'm expressing frustration that someone such as you who wants to match up a painted solution against a HP screen, a Mfg Screen held so near to the heart of so many and acclaimed as being impossible for any DIY effort to compare to, should come so close yet still be cursed with the dreaded Roller Mark Syndrome that is so easily avoided entirely by spraying.
Now the bitter pill. It will almost assuredly take more paint. You can try sanding, but really, first you must ascertain as to if the roller marks represent any textured difference (...a raised line or ridge...) or are they embedded "streaks" that show through a couple of layers of paint. Streaking is what describes a very light roller mark. Sanding is really just an attempt to prep a surface for additional coats, and those coats won't help if the roller mark is a Ridge....not simply a Streak.
So.........I'm sorry there is no easy fix for Roller Marks...there never has been. I can only hope that the marks you see can be lived with, or at some time after the screen has fully cured it can be effectively sanded down and re-coated, and you will consider such as being worthwhile to do because you have at least come to appreciate what the RS-MM LL's potential brings to the equation. The Painter should at least be willing to come back at that time to apply 2 more coats.....carefully....for his original $300 fee.