anyone feel grip is solid on wet rocks/leaves, going downward on hill, e.g.

I had a spectacular fall when stepping on wet leaves on a flat rock. I had been walking slowly on a flat trail and was totally shocked from falling at all! I learned later that as wonderful as these boots have been to me, their one flaw is their lack of grippiness on wet surfaces. 99% of the time of wearing them has been totally problem free. I wish Keen would develop alternative models that had grippier soles.

Are these boots ideal for a 3-4 day backpacking trip?

These boots should work well for a 3-4 day backpacking trip. Only thing I could add is these boots come to mid ankle, so they provide ankle support, but not major ankle support. If you are carrying a heavy pack and are going where there is a lot of fist sized rocks on the trail, you might want more ankle support. That statement is a reflection of the trail, not the boot. I have been wearing these boots for years, every day for two weeks at a time, on 10-20 miles hiking trips in southern CO. Never had a problem.

The Keen website says these run a half size small and recommends ordering a half size larger. Is this true?

This is my second pair of Keens. I also have a pair of SOLR sandals. Love them both. I wear a size 11 1/2 in all my sneakers and cowboy boots. In a Keen I wear a size 13. In both my SOLR sandals and these Targhee II hiking boots I originally ordered a size 12. Had to send them both back and get a size 13. And I can compare them sole-to-sole with any of my size 11 1/2 sneakers and they are the same length.

Are these authentic keen brand boots?

If you buy them from Davidson Shoes Inc., I can assure you they are 100 % authentic Keen boots. We buy direct from Keen. Our retail store has been in business in the same location for over 125 years. We cannot speak for any other amazon sellers. Mark Hogan President Davidson Shoes Inc.