If I complete all side-quests and all markers (nests, treasures, camps etc) what level will I be (death march)?

The only real way ATM appears to be through grinding VERY slowly as the cap is much higher than is realistically possible.

Keep in mind they have a great deal of content coming in the fall ass add on so there's room to add more upper level content.

Yea I assume the level cap is as high as it is in preperation for the expansions. Though it's possible to grind high level respawning monsters. Though I don't think all the way to 70.

How much xp do high level monsters give?

I took out a loan and repaid it, just to see how it worked. There was a net loss of 30 coins, and a gain of 13xp (not sure if this is based on level or any other factors). I did this a couple more times to see if it gave xp again, and it did. If you have a lot of money, you can try farming xp via loans at the bank. I don't know if it would be any faster than killing monsters. It would certainly take a long time either way, and be more tedious.

It's not like there's anything else to spend crowns on in the game. I'm just over 40K crowns at level 17. :(

Visiting the brothel gives XP at a conversion rate of 10 orens per 1 XP.

The game suggests that you reach at least Level 22 before starting the finale, so duh.... Someone at CDP just forgot to reset the leveling cap to 30 after some wild launch party I guess. I think the highest NPC I ever found was 35.