Photos for sure.
I believe that chain guide is a bit of a band-aid solution and would aid if the chain were jumping of the front end..
A chainwheel which is straight and true should allow shifting in the complete range
There are single front chainwheels and chainrings available, though it really will not matter.
Is the derailleur hanger bent/ has the bike fallen over?
IMO this is a very likely cause, as it would be nearly impossible to get the derailleur adjusted to
function correctly if the dropout or derailleur hanger is bent.
This is most likely, especially if the problem just started to happen without any component changes.
Is the wheel in the dropouts squarely( you mentioned that the wheel cannot be adjusted to affect chain length- I am curious why)
Any new parts, wheels etc.?
The frame alignment can also affect this. Do the string test for alignment-
Take a long piece of string around headtube and touch each rear dropout. The spacing of the string at the seat tube should
be equal on both sides.
Have you checked dropouts and other frame members to see if they have released from their proper connections?
Does the bicycle handle funny, not ride or track like it once did?
Can you ride it (safely) no handed?
Photos from side, top and and rear.