Based on the photos, it doesn’t look like the roof is too bad. Who ever installed it did you no favors though, because they forgot to flash the soil stacks, skylight bubbles, etc.
If you are on a fixed income and want to pospone the inevitable, here is what I would do.
Have the roof cored in a couple of place to see whether the roof insulation is wet.
If the insulation is in relatively good condition, and you don’t have a lot of entrapped moisture, I would have a roofing contractor install lead flashings on all of your soil stacks, skylight bubbles (but I’d actually get rid of those pieces of crap).
Repair your fishmouths, open seams, blisters, and major ridging.
Install base flashing around large skylight curbs, chimney, etc., but I’d probably let the tie-in to the cedar shake slide for now.
Strip-in all the repairs made with roofing cement with new modified bitumen membrane.
Coat the roof with an elastometric membrane.
Now, the more I look at the photos, the more it looks like you may have some poor adhesion between the roof membrane and substrate. I can’t judge from the photos, but if it is bad you may be better off replacing the roof.
One thing for sure, whoever installed the original roof should not be allowed back onto your roof to install the next one. Anyone that would install a mod. bit. roof without flashings is either a moron, or is looking for repeat business in the near future.
The easy answer is to replace your roof, but if you go the repair route you may be able to buy a few years before you have to replace the existing.
More photos would help, especially in the area where it looks like there is a lot of ridging and disbonded membrane. Have someone core the roof, and that can ultimately help you make an informed decision.