Re: About govenors in cities
yes. I believe your income and public order are penalised for not having a governor (so even a governor with no bonii is better than no governor).
Re: About govenors in cities
actually you need only one military unit to cancel out the public order minus. Income is not penalised, only thing that you lose are the various bonuses and maintenance income your FM would give when in a city.
Re: About govenors in cities
You can leave cities ungoverned (but not ungarrisoned), that's no problem. You will always have one or the other minor town close to your capital that does not desperatly needs an expert administrator.
I would not always go for client rulers because you can't get rid of them for about 30 years, and when you are low of family members you are often bombarded with children the next years who are asking for provinces to play king in some 15 or 20 years later. You should rather consider to let your armies fight without generals because that might bring you some "man of the hour" adaptions.