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This appears to show that the glycans shielding the virus are built from mannose.

Question: Does all of this mannose come from the diet, or is it built in the body from something else?

If all of it comes from the diet, would altering the diet so that it produces less mannose after digestion help control both this virus and the others mentioned?

If not, does this make the various protein sites where the glycans bind a good target for binding something to block glycan attachment?

I get the drift of your questions as being does altering glycosylation offer any therapeutic value in fighting Covid-19? I doubt it since it's so integral to so many normal cellular process from the springiness of our cartilage to intercellular signalling to forming IgA & IgG antibodies.

"Glycosylation is a common modification of proteins and lipids that involves non-templated dynamic and complex processes. Glycans have multiple crucial roles in cellular responses to environmental stimuli as well as cellular growth and differentiation; specific changes in glycan composition are directly linked to many diseases." Take a look at Figures 1-7:
https://www.nature.com/articles/s41581-019-0129-4

Take a look at the Metabolism biochemical pathway diagram on the mannose page: https://en.wikipedia.org/wiki/Mannose
Everything ending in -ase is an enzyme and -ose is a sugar. If one were deficient in mannose your body would up-regulate certain pathways to make mannose from glucose, fructose, galactose or glycogen (stored in fat cells).

Problem I'm worried about is that if a drug is developed in silico for the bare viral protein but in the wild it was covered in wavy sugary hairs that hid its binding site then it would be useless. The scientists at Scripps are among the best in the world and they'll plan for this. It significantly reduces available target sites.

The places where proteins or glycans connect to cell walls are called GPI anchors.

Glycosylphosphatidylinositol (GPI) Anchors: Biochemistry and Cell Biology: Introduction to a Thematic Review Series

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4689333/

Human metabolism uses some on human cell walls, so elimination all of them is not a good idea.

A GPI anchor contains mannose and a phospholipid.

World's dominant strain of coronavirus 'is 10 TIMES more infectious than the one that jumped to humans in China' because it mutated so its vital spike protein doesn't snap as often in the body, scientists say

A mutated version of the coronavirus that has gripped Europe and the West is more infectious because it doesn't break as often inside the body, a study has found. Researchers at The Scripps Research Institute in Florida say the 'spike protein' that the virus uses to attach to cells in the airways has adapted since January.

http://a.msn.com/05/en-us/BB167e7l?ocid=se

Study finds hydroxychloroquine helped coronavirus patients survive better

A surprising new study found that the controversial antimalarial drug hydroxychloroquine helped patients survive better in the hospital.

http://a.msn.com/05/en-us/BB16hifu?ocid=se