jxself, thanks for your detailed answers!
this apt string worked for me to setup alternative archive as software source
(for trisquel 5.5):
deb http://mirrors.ukfast.co.uk/sites/trisquel.info brigantia main
unlike trisquel's official archive and other mirrors this one seems to
contain all (or at least most) packages for "short term support" vesions of
trisquel
my next struggle is to run trisquel's desktop in half-resolution (for
exxample 960x540 resolution on 1920x1080 TFT panel) without blurring, as
described here:
https://forums.linuxmint.com/viewtopic.php?t=159064
I managed to rebuild xrandr with lines 2886 - 2888 of its source code deleted
(so that it would always use "nearest" filtering method).
Also can run it with ./xrandr command except it still seems blurry (or maybe
my TN TFT panel is too low quality to clearly see the difference?).
Any ideas on how to test it in some non-subjective unambiguous way? (without
using TFT TV sets with huge low-resoltion screens - I do not have easy access
to these...)
