"Need I stick to the lines?", and other road markings and traffic signs

I’ve looked carefully at THC.  At 109 it only says “Make sure you … act on … road markings”.  At 127-132 is mentioned “should not” and “MUST NOT” once each, but “Do not” also once.  For roundabouts specifically, at 186, for intermediate exits it says “select the most appropriate lane on approach [and] stay in that lane until you need to alter course to exit”; “should” is used only regarding cyclists and horses.  In short, I remain unclear what we're REQUIRED to do (and've been since 1981).

I believe that a road roundel with a red outline counts as mandatory, but I’ve only seen those on the private roads of Cranfield University.  (I know some local authorities use red on road-marked warnings, such as "Children in the Road").

Some non-circular signs can refer to speed limits, which is confusing.  And circular ones may appear with a grey or a green outline (at the start of 20 mph zones), presumably just advising behaviour.

TSRGD2016 seems to imply that the size of the traffic speed roundel is only for visibility, and not to mark start of a new zone (though that often coincides).  Certainly, “speed zone gates” are no longer mandatory following a recent Minister for Transport legislating to reduce urban clutter: only a roundel at EITHER side of the road has been necessary since 2016.  That said, as 20min zones expand, certainly in Birmingham there has become a confusion of zone boundaries due to older roundel pairs being left in situ (as well as repeater signs sometimes being over-spaced).

Also confusing, deflection arrows on lane 1 as you come up from the A38 Queensway underpass in Birmingham, do not mean you MUST merge with lane 2 of the new road on your left.  And some roundabouts strangely have a single broken line across the WHOLE mouth of each exit/approach road, rather than just each approach half (this also occurs at some 2-way minor road junctions).

Related to that, how do we recognise WHICH solid white longitudinal lines can be crossed if it's not an emergency?  The thickest are for Bus Lanes.  The thinnest are for parking spaces.  Is it therefore just the MEDIUM thickness ones (along centre of carriageway or its edges) which we shouldn't?

All in all, these inconsistencies make general rules harder for novice drivers.