It is possible to have both a housecarl and stewart at the same site. i had turn one of the sidekicks into a stewart at one of the houses in the wild. then became thane at the capital of that hold. the housecarl assign to that house showed up. even saw him inside the house.

something to try if you want children to live in one of the houses in the wild. two bodyguards to protect them.

As far as I can see, yes. Regardless, if you become a Thane in a hold where you build a Hearthfire house (Dawnstar, Falkreath, Morthal), you will have a housecarl assigned to you. If you do NOT take that housecarl on as a Follower, then he/she is obligated to protect your property -- the house -- in that hold. Beyond the housecarl, you have the option to invite one of any number of other NPCs that you interact with to be the Steward of a house that you own. A wandering bard for example. In such a case, you would have both a Steward and a housecarl staying at the same house. (Also a hired bard and carriage driver as well are options.)

Yes you can.

In addition, if you marry a housecarl, that'll give you three housecarl a at one property. I usually end up doing that. No one stands a chance

Hi OP - a gentle and kindly correction for your post: 'stewart' is a person's name, the title of your servant in 'steward'.