Most times, yes. For nonfiction, the developmental edit will ensure that you are addressing the pertinent topics, that you aren't veering too far off the main course or leaving any holes, and most importantly, it will help organize the content so the reader receives your message in a clear, concise way.
For fiction, most authors require a developmental edit. It is rare that a novelist creates a story that is entirely complete. Most developmental edits in fiction help the author create well-rounded characters with plausible plots weaved within a journey that entertains and moves the reader.
I recommend authors show their edited manuscripts to a handful of beta readers or a critique partner before finalizing the book. This is especially helpful in fiction and memoir.
Yes. I can provide feedback on query letters and proposals. My standard fee is $250.
There is no simple answer to this question, but in general terms, I recommend a debut author who is deciding between going the traditional route and self-publishing to do the following:
First, shop the proposal to a dozen literary agents. If an agent agrees to rep you, tell them upfront that you are giving them a six month window to shop the book but that after six months, if no publishing house has showed interest, you intend to pursue independently publishing. This allows you the peace of mind knowing you tried the traditional route.
Do not let rejection from traditional publishing convince you that you are not a good writer or that your book isn't worth publishing. Many wonderful books are rejected by a traditional publisher because they wouldn't be able to sell enough units in the first 12 months to make the effort financially worth it. It's personal for you, but it's business for them. This is not a reflection on your skill or talent.
Having a platform is essential in nonfiction. When you have a strong platform, it assures the publishing house that you have a tribe of established readers who you can market your book to. This lessens their financial risk and provides more certainty that they will be able to sell enough units to earn back the amount they advanced you and maybe even be able to send you a royalty check.
In fiction, it's important, but contracts can be offered for books even if the author isn't well-established. The main difference is that fiction has a readership that is usually less concerned with the author name and their expertise in a particular field and instead will select based on genre types like historical or romance. Having said that, debut authors are rarely contracted. The amount of effort and time it takes to launch a new voice in books isn't usually a sound investment for the publisher. This is, in large part, why self-publishing has grown so much in the past several years.
For novelists and memoirists, these are must-reads:
Save the Cat by Blake Snyder
Writing Fiction by Janet Burroway
Pixar Storytelling by Dean Movshovitz
The Writer's Journey by Christopher Vogler
Writing the Breakout Novel by Donald Maass