Emulate Stack Exchange as much as possible
Stack Exchange shares most of the design challenges that you are faced with and there solution has already been tested by millions of users. That doesn't mean it will be perfect but it is a good start. There are some notable differences between your situation and Stack Exchange,
- What you call categories, Stack Exchange calls tags.
- Your categories have a layer of hierarchy, Stack Exchange tags do not.
- Stack Exchange has one type of submission - a question, while you have three types - questions, posts, and events.
Categories and subcategories
I agree that traversing the category hierarchy before writing a post would be too much work. Consider associating posts and subcategories the way Stack Exchange treats adding tags to questions. Have your three submission options as top-level actions and let the user select the subcategories to file it under on the submission page. Note that I'm assuming that submissions must be associated with subcategories (and by extension a category) and can't be associated with a category only.
Stack Exchange allows the user to perform a search of tags on the question submission page as shown below.
If you make your subcategories accessible in this way it would make it easier for the user to find existing subcategories. The description shown for tags here is essential as well as it helps the user evaluate the fitness of the tag. However, this doesn't solve the problem of determining which of all the subcategories is most relevant. To help with this you may want to include related subcategories in your search results not just alphabetical matches.
If you think the user may become familiar with the category hierarchy over time you may want to give them the option to use category as a search filter. You may need to design keyword operators for this, to reduce the UI complexity. For example if you use ':' as a keyword operator for filtering results by category I can enter the search term - 'sport:ba' which will show results like 'baseball', 'bat' and related subcategories like 'glove'.
Filing a post under a subcategory seems to be an important requirement for you, so, I would consider placing the field for subcategory selection before the field(s) for writing the post.
User submissions and confusion
The degree of confusion your users will face is proportional to the degree of ambiguity in your interface. You mention that Stack Overflow users are clear on what is expected when submitting a question. I think this is because, Stack Overflow educates the user on how the site works, both in their onboarding and in the interface.
The interface for submitting a question, shown below, has all the cues to communicate what is expected to the user - a separate and labeled field for the title and an area for describing it. To be fair, the description area has no title or placeholder text for clarity. But, the 150 character limit on the title forces the user to eventually reach for the description field if it isn't immediately obvious.
Placeholders may not be the best solution for educating a user on what information is expected according to the Nielsen Norman Group.
Summary: Placeholder text within a form field makes it difficult for people to remember what information belongs in a field, and to check for and fix errors. It also poses additional burdens for users with visual and cognitive impairments.
So you may want to consider placing field hints outside the field unlike Stack Exchange.
Consider improving the button texts for your submission pages. You can again copy from the Stack Exchange playbook and call Add question - Ask question. Post content is very vague so once you are clear on the type of content the user should post you can rename this. Remember, you need to guide the user if you want them to put content in the correct place.