RE: DO RESTEEMING AND MENTIONING actually work well? Are those efficient ways of building reach and exposure on Steemit?

I think you point out a very important fact that is problematic here on steemit:

How to communicate with people on steem ?

You pointed out three different options and none of them seems really satisfactory:

Memos : By many considered as wallet spam, it is quite tricky and not unexpensive to use. If people are playing steem based games, such memos are lost in the wallet transactions. Still it is possible to use memos if people really agreed to recieve them in the first place. Maybe an idea would be to create an app where people could register/unregister from memos
Resteems: People are not watching their feed very much so resteems are not really a successfull way to generate engagement in a post.
Mentions: Most users don't react to mentions because they use tools (steemit, steemd) where mentions are not visible.

Other solutions to communicate with users:

Using different apps: I've downloaded the Partiko app recently and now I get a notification each time somebody sends me a memo or mentions me in a post. These tools have a much higher impact when users get a notification to their phone.
Using internet marketing tools: Services like mailchimp allow you to create lists and send e-mails to your list. The advantage of using external e-mail services like that is that you can make sure that you comply with laws before you send the mails. Mailchimp offers free opt-in pages where people can register for your newsletter. So whenever we write an interesting post on steem we could send an e-mail to the list and put the link to the post in it.

Conclusion

I believe that the communication between steem users is somewhat complicated and not so easy to achieve. The means to communicate are not successfull for all users because we all use the steem blockchain in different ways, with different apps and interfaces. In the end you get the users that read your posts according to the means that you use to prompte them. If you use memos it is the people reading those memos that will come to your post...

Best regards,
Achim