No, they're not interchangeable. I agree it's confusing. Here's how I understand it; hopefully it's correct:
If you want to say "I like milk", you use "le", as in, "J'aime le lait," but if you want to say "I have milk" you use "du" as in "J'ai du lait". (Or if the noun is feminine, like the word for "meat" which is "viande", it's "J'aime la viande" vs "J'ai de la viande".) I guess it's because when you like milk, you are talking about milk in general, so you use "le lait" (or "la viande" for meat). But when you have milk, you can't say that milk in general belongs to you. Only a certain quantity of milk belongs to you, so you use "du lait" (or "de la viande" for meat).
Very good guess, you've noticed that you can't say "J'aime de la viande".
Use of the partitive = to mean you have an indefinite quantity.
The partitive in French are de+ definite article ("du" being the mandatory contraction of "de"+"le", it's a partitive)
Partitive are translated in English with "some", or no article. Example:
J'ai du lait = I have some milk = I have milk.
If I say J'ai le lait = It means you have THE milk, a definite milk, for instance the milk you've bought.
The "aimer" verb is an exeption to know. It won't accept the partitive. The rule is that you can't use any partitive with this verb. So, you will only say "J'aime le fromage", "J'aime les fromages".
It's for indefinite quantity here.
But, you are perfectly right, it can be used with abstract nouns. (that's one of the rules of usages for partitives)
J'ai de la chance (luck), j'ai du bonheur (happiness), j'ai de l'énergie, etc...
In French grammar, thinking in terms of countable/uncontable nouns is wrong, but thinking in terms of indefinite quantity or abstract nouns is right.
I don't know for sure, but i think the French would assume you mean plural by context clues, such as using "sont" and other words to support the sound.
As for gender, if you are talking about someone, they should be able to assume that person is a boy or a girl (also based on context clues and the name of that person).
Also, if you are talking about someTHING, the french person should know if the word is masculine or feminine. And if they don't, well... does it even matter??? In all honesty i don't believe one will criticize you for improper use of articles.
Just try to pronounce the word and its article over and over again to get them both in your mind. It's the least you can do to help with those little articles that can make an answer wrong, at least on Duolingo.
I hope this was a little helpful! I am a beginner too :)