Blog a success but hitting a wall - What to do?

Hi,

I'm one of those people signing up here years ago and never posting, since there is SO much to read here, I always felt stupid for even thinking about asking questions.
Until now.
What changed? - I have hit a wall and want to get over, under or around it and grow.

Situation:
I have a WP-blog since 2011 in the food-niche. Since 2013 I started to read up on SEO and make all the mistakes that can be made, but since 2014 I make some real money with the blog, mostly through affiliate links (non-amazon) and Adsense. My blog currently has ~500 posts.
(With 'real money' I mean I can feed my family three times a day from the income I generate with this blog.)
My traffic went up in 2014 to reach ~45k visitors/month, 99% from organic traffic (Google), ranking for some specific keywords that also have big selling potential.

Starting in February 2015 my traffic started to decline, at first I thought it must've been a new plugin I installed that allowes for tabs (accordion) to be displayd on my posts and pages, I removed them all but traffic didn't come back and is now at ~50% of what it used to be.

The weird thing: income kept rising, and June 2015 was my best month ever income-wise. The RPM has gone up significantly, but I am not sure why, I suspect that the drop in traffic is not affecting my income because the lost traffic was traffic going to recipes-posts, and those do not convert well on Adsense, let alone Affiliate-sales. So I think the traffic that still comes in visits my posts on cutlery/kitchenware, and those have the affiliate-links and product feeds, and those sell.

Now after wondering for a long time about what caused the drop in traffic (reading all kinds of tips; to check for spam, etc.), I today read an interesting comment by SantiSantana here on the Warrior forum:
So since he mentions 2015, and Hummingbird, I wonder now what I missed, and wondered if any of you got some insight on what was going well for my traffic in 2014 and suddenly not so well in 2015. What are these 3 things Hummingbird wants me to do?

And then comes the other thing:
Making money with blogging is all about content, good quality, original content. So I write 1 article every 2 days (>400 words) with researched content, title chosen on keyword, original images added. I push it to FB, Pinterest, Twitter (not all the time).
The other day I find a niche-related article and quote the content partially, linking to the original source, and adding my insights on the subject.
So my blog gets fresh content daily.
Next to that people do comment on my articles, share my recipes on social media.

But what I lack, and have never understood, is how to get backlinks. Because according to some of the backlink-tools, my backlink profile is very weak.

I have read tons of blogs, guides, how-to's and posts here on the forum and came up with this list:

1. Social bookmarking
2. Quora (answer questions)
3. Yahoo Answer (answer questions)
4. Issue Press Releases (after you have done something newsworthy)
5. Guest posting
6. Info graphics
7. Forum Posting
8. Web 2.0 (social media)
9. Slide sharing
10. Broken Link building
11. Files sharing
12. Local directory submission
13. Blog commenting

Now I started with making comments on blogs (13), which seem to never be a do-follow link (as confirmed by some members here), I made good comments, on topic, helpful, etc. I get some direct traffic from them still, but maybe 10 visits per week.
I also do 8: Social media, although it seems FB is not great for me, people do not follow me at all. Pinterest works better, considering the audience there (women, 30s) and the shareability of food-images (my niche), but not more than 5 visits/day maybe.
Forum posting, #7, I do too, but I am not sure if the links in there are do-follow links, I get some direct traffic from it though.
Answering QA(2-3): I am not in the US and my audience is also EU so I am answering questions on another popukair QA-site, getting some traffic from there, and the links are good according to google.
Infographics (6): I made a good one, but sites that are willing to share it for me want a monthly fee, and I think that's rather silly, so the infographic lives inside the content it is about.

So what I don't understand is:
What is social bookmarking, if it is not sharing content on social media?
Issuing a press release: what could I release? Is doing a survey and releasing the outcome a good idea? Where to release this press-release to? Where to send it?
What is slide sharing? Do I make a slide from images and share that? To whom?
Broken link building: I have tried this, but go no response nor backlinks - do I need to find hundreds of broken links and contact the site-host?
Files sharing - what files do you share, and what is in them? Do I compile an ebook from content I have written over the years?
Local directory submission: my site is not 'local' orientated, its about cooking, food and kitchenware, what directory do I submit to?

Another thing that I can't seem to figure out:

List building. Apparently it is crucial to 'build a list'.
So I got some e-mail addresses from my visitors, then what?
Send them an email once X days with....what? What would I put in the email without it being considered spam?
I have to ask permission to send out affiliate-offers to my 'list' from every affiliate I promote items for, because of EU-regulations, this is very strictly supervised now. So what else do I send them?

As you see, I am in this for a couple of years but have clearly not understood a lot of things, and I want to try and catch up, because I feel the success slipping through my fingers and I don't know how to stop it. I hope to get some tips or hints even on what to look for, how to get my traffic back, because income is good now, but with traffic fading this won't last.

I sincerely appreciate any reply to writing, and feel honored you read the whole thing.

-Frits