Is it possible for an editor to review the paper directly without inviting another reviewer if the editor feels familiar with the content studied in the paper?
Depending on the rules of a given journal, an editor could take on the review task themselves or not. Some will permit it and so some editors will also review. Most will want some independent advice, I think.
I also think that editors are most likely to take on the obvious and simple cases, however. So a long delay is most likely (not necessarily) something else.
There are a lot of reasons for any delay. This could be all sorts of things. The preferred reviewer might be overloaded at the moment. The editor might have been having a hard time finding appropriate reviewers. There might have been fast reviews and it bounced back quickly but the editor hasn't made a decision. The editor may not have found a "suitable" slot for the paper and is holding off sending acceptance. There may be a negative review and the editor is seeking clarification. Lots of possibilities.
And "quite long time" feels different to you than it does to the editor. It can be frustrating. Either ask for a status update or be patient. But "weeks" is not very long. For some journals "months" isn't excessive.
Yes, the editor can review a paper directly without inviting another reviewer. It's possible journal policies can require at least two reviewers, in which case the editor would have to get a second reviewer; however it's also possible the editor can ignore this policy if they feel it's justified.
To answer the situation in your comment: "With editor" doesn't mean the editor is reviewing the paper. In my experience it's much more common the editor is simply not very active (or very busy) and isn't inviting reviewers. If it's been a long time I would suggest asking for a status update.