I have custom forms for my list.

How to change default display/edit/new forms to those files with "-ifs.aspx" ?
Even better, can I do this without using SharePoint Designer? Is it possible to set default forms from web UI or maybe with PowerShell?
I have custom forms for my list.

How to change default display/edit/new forms to those files with "-ifs.aspx" ?
Even better, can I do this without using SharePoint Designer? Is it possible to set default forms from web UI or maybe with PowerShell?
You can set it as default from SharePoint Designer 2013 as well. You need to select the form and then it will enable the "Set as default" button control in the Ribbon.
Using powershell, should be something like:
$web = Get-SPWeb http://path/to/web
$list = $web.GetList("/server/relative/path/Lists/ListUrl")
$list.DefaultDisplayFormUrl = "/server/relative/path/Lists/ListUrl/displayifs.aspx"
$list.DefaultEditFormUrl = "/server/relative/path/Lists/ListUrl/editifs.aspx"
$list.DefaultNewFormUrl = "/server/relative/path/Lists/ListUrl/newifs.aspx"
You cannot change these properties from the UI.