grammatical number - Can an “s”-form plural follow an “s”-form possessive?

This is best described via an example. I believe this might be technically correct, but sounds clumsy:

You need to look through all the chemicals shelves

There are multiple shelves, of type chemical. There is a chemicals shelf times many/plural. On the other hand you could have:

You need to look through all the chemical shelves

This sounds better, but to me implies that the shelves themselves are chemicals. Obviously context factors in, in that most people would know that the shelves are not chemicals.

Answer

Plural modifiers are entirely acceptable.

Your doubt about the validity of the plural form comes from a rule in the past that noun modifiers had to be singular (apple tree, vegetable soup, rubber chicken) but today this is not an absolute and there are many examples of plural noun modifiers in everyday use, for example, parts departments, schools superintendents and options markets.

In Quirk et al., A Comprehensive Grammar of the English Language, the following situations are listed where a plural modifier may be used:

1. 
   

   the singular form might lead to ambiguity
   an Arts degree (a degree in the humanities) as opposed to an art degree (a degree in fine art)

   
   


2. 
   

   there is no singular form of a noun (in pluralia tantum)
   a customs officer

   
   


3. 
   

   there is a need to denote variety
   a soft drinks manufacturer [but] a car manufacturer

   
   


4. 
   

   a topical issue comes forth, often in newspaper stories. Quirk cites examples of Watergate reporting from newspapers:
   the tapes issue
   the tapes compromise
   the Watergate tapes affair
   the White House tapes mystery and other examples, including jobs cut.

   
   

In your case, chemicals shelf might be used because of the variety of chemicals. However, I disagree about the potential for confusion. Taking the example of the electronic engineer, when could that ever be interpreted as some sort of robotic device? No such thing even exists. The same applies to mechanical engineer but in that case there's no alternative.