It would be interesting to find out how many of the 400 people who downloaded your giveaway went on to read the book, and to buy anything else by you. Is there any way you can find that out?
From what little I remember from my marketing and advertising days, mailings which aren't targeted usually have a positive response rate of under 2%. Often less than half of one per cent even respond to those mail outs, and a response isn't a purchase, it's an enquiry. So ads which aren't targeted are very likely to have a very low response rate.
If a publisher sends out 200 ARCs (I'm talking about printed copies here, not e-copies, which might make a difference) it would be good to get 5-10 reviews as a result. ARCs aren't just sent out to reviewers, though: they're also sent out to buyers in book shops, etc., so that makes a difference.
Book signings and author events only really work if the author is well known, because few readers will turn up to see someone they've never heard of.
Yes, this all sounds very negative, with lots of effort for little reward. The point is, though, that all that effort builds up. Those ARCs might not garner many reviews but they do foster an awareness of the author. The next book they write might get more reviews. The reviews that appear build on an author's profile. Some time down the line a reader will see a review and remember they'd meant to buy that author's previous book and buy a few from the backlist.
What you try to achieve with a book launch is an increased awareness of the author as a brand, as well as stirring up an interest in the book you're launching. You might not get lots of sales for all the ads you pay for but the potential readers who see that ad might then also read a review, or come across you on social media, and it's the accumulation of those things that generally lead to sales.
So it's really hard to pinpoint what exactly works, and what sells books. Because it's all of the work we do, added together.
Which is why trade publishers employ people to do all sorts of sales and marketing work.