I have now purchased Moon+ Reader Pro (€3.80) and Mantano Premium (€8.36) and also ezPDF Reader (€3.00), which seems to be the most reputable PDF app on Android, and "Android's answer" to GoodReader in iOS.
If I notice any features in those readers worth considering for Marvin implementation, I will post them here or to
Marvin's GitHub. For example, Mantano's
Notes Manager sounds intriguing, at least in
theory: it allows you to view and manage your annotations from
various books on a single pane.
A couple of observations from the first test runs of the premium versions of Moon+ and Mantano...
A feature where Moon+ Pro is superior over the current Marvin is
vertical scrolling. And, in many ways, Moon+ Pro is even
more configurable and customisable than the current Marvin.
But Moon+ Pro's
main advantage over Marvin currently, as I see it, are its plentiful (
unlimited, not only 3 like in Marvin today!)
themes, optionally employing
background textures (along with, again
optionally, many
other formating settings, not only font & background
colours), which are absolutely stunning. Regarding Moon+ Pro's themes, the current thread would need to be renamed from
They Are Soooo UGLY! to
They Are Soooo BEAUTIFUL! 
Please don't tell me, when you look at the screenshots below, that we do
not need background textures in Marvin. Of course we do; I find solid-colour backgrounds so incredibly boring compared to textures. The "old book" theme/texture in Moon+ Pro looks even more amazing in real life, when displayed on the new Nexus 7's ultra-sharp screen, than it looks here in screenshots. The first screenshot below is the default "old book" theme (Day 5) in Moon+ Pro; the screenshot below that, is the same theme as further customised by me. The only thing spoiling the look for me, are the too large 1st-line indents for paragraphs; but there appears to be, indeed, no way in Moon+ Pro to adjust the size of the 1st-line indents.
On the other hand, Moon+ Pro is clearly inferior to Marvin when it comes to
fidelity of rendering e-books. Moon+ Pro totally mangles the display of my
EPUB test file. It's as if Moon+ Pro was arbitrarily selecting
any style it prefers for displaying the test file -- disregarding the CSS employed in the e-book's code. Mantano Premium displays the test file better, but with flaws, too: neither Moon+ Pro, nor Mantano Premium seem capable of displaying
small caps. What they do
better than Marvin, is that they at least preserve the publisher-specified
font colours (see
GitHub #99).
Regarding
Mantano Premium, its interface seems chaotic. Although the app costs €8.36 -- roughly double the price of Marvin for iPhone -- I
still can't see any
paragraph spacing setting in Mantano Premium. And although I specified zero page margins both vertically and horizontally, Mantano still leaves a fairly wide margin along the 4 screen edges in some (why only
some?!) e-books. Mantano seems to be enforcing "publisher settings" for
some EPUBs, with no easy switch provided for clearly distinguishing between publisher and user formatting preferences.
My first ad hoc conclusion would be that, despite the lower price and problems with rendering fidelity, Moon+ Reader seems to be the better choice from among the two.
