Mine are opened on a platform. For some reason, mobs would attack the torches or a stair case, but leave the portal itself alone.

Yes they are attacked.
Easyest way to protect them (except in the mountains) is to make a 5m high plateau (with trolls arround even more)

Yes they can be attacked. I am lazy and only sometimes house mine. Many moons ago I was running from a troll and jumped thru a portal. By time I arrived on far size the portal back no longer worked because the troll had smashed it behind me.

They seldom do, but yeah, enemies do attack portals when they come close.

Normally if you're not there, you can leave them in the open and just repair them once in awhile. But when there are raids or troll attacks, it's best you lure them as far away as you can.

Yes, they are an aggro on sight item like other "player base" objects. If an aggressive mob sees it they'll attempt to destroy it.
Bear in mind if no one is remotely near nothing can happen to it, but they should always be somewhere safe so you don't come through a portal to a troll in your face.

In my experience it seems mobs prioritize attacking some built structure over others. While boat undoubtedly have the highest priority, portals seem to be very low on their list.
During the bronze phase I often lay down a workbench, a chest and a portal outside in the grand open. The former two often get attacked, never the portal

i usually build a small shack around it. a little 2x2x2 with a roof on it is adequate.

for a portal to a swamp, i try to find a shoreline crypt, and i build on top of that and build stairs off the back side. Crypt entrance structure is invincible, and most things wont get up to the structure on top of it. and being on the shore, less chance of being jumped by enemies.

I have mine outside with no issues. I do put three walls and a roof up, open front.

yes they can be attacked. i would suggest building a wall around :)

Only if you leave enemies near them. I never house my portals.