So far, I understand that there are Followers, Mercenaries, Dogs, Horses and friendly NPC's.

A mercenary becomes a follower upon being hired, but apparently so does a dog?

Horses, no matter how many of them you own, cannot be gifted away or lend to other people, and no other people in the game that I have come across so far, that are willing to follow you do in fact own horses.

Followers have many more capabilities than they used to have, which is cool, but not being able to see their sheets (and there being no character sheets in the game) makes it difficult to pick a companion before going on a quest.

In relation to dogs, I had wished that the dogs would be "summonable", or that you could indefinitely park them at your residences as guard dogs or what not. It seems strange that the dog still considers.. Well, either markareth or the wilderness to be it's home, after you buy it/find it. It seems silly to me that you have to buy it again after leaving it alone for 3 days. That same dog.

It would also be swell if they were "renameable", and to be honest, that would be cool for horses too.

Travelling with a companion would be easier if you could make him/her mount a horse, and if there was a way to instruct certain tactics to him/her, such as "in case of being attacked while riding, keep riding" or, "keep long distance from me".

Despite all this, I find the game hard to beat on the tougher levels of difficulty when I'm alone. Any tips on companions and how to make them work, despite their shortcomings?

I like how you can boost their equipment at least, and instruct them to interact with the surroundings. I just wish they had more options. "Guard this door" or "follow that person" or some such.

Anyway to make good use of that ONE companion your allowed?