File & folder sharing with privileges on my network does not work. What is wrong?

My Timecapsule is creating a network between my iMac with OS 10.7.5 and my Macbook Pro Retina OS 10.9.2. I am trying to setup a folder on my iMac with controlled access privileges to work as a server for others using the Macbook as a client. I followed different tutorials to the letter, but it did not work, I could not get the connection. Then I called the Apple help line. It turns out that just setting it up with a 'sharing only' account etc. in 'sharing ' in the 'system preferences' is not enough. One also has to set up the same priviliges in the 'get info' window of the folder directly at the same time...so a double setup. They could not explain properly why and there is no mention of this whatsoever in any tutorial I found on the internet. Anyway, I tried that and it still did not work. When I add the user in the 'get info' window and press enter, suddenly the user does not show up in the privilege box with the user name but instead a line saying something like "searching for user" and something small dudeling next to it......I called Apple again. After two agonizing hours on the phone where I was treated like an imbecile it turns out one has to somehow go through the address book ??? Why, I have no clue and how, I do not remember, but suddenly it worked.

Anyway, now two month later I had to add a new user account to my Macbook and I need to give it access to my server folder on my iMac. Well same problem again. I cannot remeber what we did 2 month ago an cannot call Apple, because my service period has expired. I tried myself and got the dudle in the folders 'get info' window again. Then by mistake I deleted myself as the only administrator account on the Imac from the access privileges to that shaered server folder. When I tried to put my account back into the folders privilege settings I got the same message and the dudeling thingy there too. Frustrated I thought it is best to just re-start the computer.......that was the end of it all: Now the startup window does not show my account name and icon anymore, just two lines to fill in the account name and password. The upper line has an arrow pointing left, the lower for the password an arrow pointing right. I filled in the account name and the password....the dudle circle comes and nothing happens, it seems to crash. When I click on the arrows the sign-in lines disappear, only the three buttons for going to sleep, re-start and turn off at the bottom are the only items left on the screen. I tried 10 times to no avail. This is turning into a nightmare!! My account is the only user account on this iMac.......

1. What am I doing wrong? Or what is wrong with my iMac?
2. Why is there nothing mentioned about these issues in the tutorials or anywhere else? Why do the tutorials say it is so easy, when even the staff on Apple's helpline cannot figure it out without having lengthy side conferences with their co-workers and managers???
3. Does anybody know what I have to do again with the address book to add this external 'sharing only' user account to the folder's privilege settings?
4. I assumme I have to re-boot my computer tomorrow and restore it from the Time Capsule. How do I do that? I cannot log in????
5. With the setup I had working for the last two month there were still issues with acces conflict to new subfolders and files inside the shared folder between me and the Macbook account user. I have read that the solution is to create a group and give the group access privileges to the folder. However, I could not find any details on the exact steps to follow. I need the exact steps, otherwise this will turn into the next nightmare. Where can I find this info.

I would really appreciate your help with answers and solutions to these complex issues

Thank you