A few comments:
The Fin may have seen only 18 ascents, but on Castle Rock Spire there have been something on the order of a few dozen parties in the past 50 years.
Heat: sure it can be bad, but we were there June 7-9 in 2002 and it was actually just fine. Perfect temperatures actually. Craig Clarence & Co. hit a heat wave, but we missed it. If you go much earlier, you may have to concern yourselves with taking some sharp metal thingies to deal with potential snow/ice in the gully leading up to CRS (probably not as much of an issue with The Fin). A friend tried to climb CRS in mid May a few years back and spent 4 hrs climbing the ice in the gully. They had just one ice axe and no crampons and this doomed their attempt. In early June 2002, it was possible to avoid the ice. Of course this is also a function of the snowfall for the year. And in my case, an ice axe was still handy for the approach as a walking stick, to beat back the poison oak, to help hang the food at camp, and to downclimb an 80 degree dirt slope we blundered into on the way out.
We used a combination of DEET and Black Flag (on our clothes) and had only one tick incident. Poison oak we dealt with as per our susceptibilities. My friend essentially ignored it, despite falling in it, and had no rash. On a scale of 1 to 10, I'm probably an 8 in terms of being allergic to the stuff. I wore a Tyvek biohazard suit, stripped naked after we passed the last bit of poison oak and washed my entire body and clothes in Tecnu, and then repeated this for the trip out. I got a tiny bit of poison oak in the middle of my back. We also saw (and nearly stepped on) one small 4 foot rattlesnake.
We only climbed CRS, but The Fin is an absolutely beautiful, stunning wall of granite. I was transfixed by it and actually got dizzy looking at it during our climb.