Hi, Chris.
Re the positive flashes. No, it does not make any difference to me what polarity the lightning is when photographing - well, it never used to be! The misonception is that all big dog leg or similar flashes are positive in polarity because of what you mentioned, being at the top of the storm. Sure there is a positive pool aloft, but where does it originate inside the storm? Did a negative leader shoot vertical and then outward? Was it a negative upward leader that connected? And so on...not knowing 'where' the leader started from within the cloud is something I posed to Prof Earl Williams. In the easrly days i sent him some images of what i thought was positive lightning - he wanted proof or data to show him they were positive, because they are not all that frequent in storms. That's not to say storms don't produce them, they do, just not that regularly and if they did they could be intracloud flashes or flashes you can't because of cloud or rain...remember that this does not necessarily mean that some of these large leaping bolts are not positive...they def can be!
You can tell by looking at the image what type it could be, there are things called M components, the structure of the branching, the tips of the leaders, their actual direction...so indeed, one can tell but it takes a little expertise to do that.
The hardest thing is to break the misinterpretation of what people are seeing. A lot of research has been done since the early days and people are kinda stuck in that mindset that everything coming from the upper regions is positive, the photos i added actually show the flash exiting the middle portion of the storm which gave it away. I've taken heaps of positive lightning, as have most people, we just don't know it. We all think that cloud based lightning is negative...yeah most often it is, but....did the leader start at the base of the storm (+) or inside it (-)...the same problem sticks its head out.
Your images show two flashes and they are grounding in the same location, not the same spot. It's not uncommon for this to happen. A location will get belted by several strikes, but in the same actual spot by millimeters, unlikely. When i replied to lightning hitting the same thing twice, yes it does, but with return strokes first and then can be hit again several seconds later...these are usually communication towers and the like which are high targets because their charge is enhanced at their tips. Objects at ground level are rarely hit more than once because there are more targets to choose from but most are at the same level height wise. If you do see lightning hitting the same region more than once that is just showing you an enhanced highly charged region as the storm moves over the top of it. A thunderstorm does that, it acts like a generator to things it passes over. Why some objects produce upward leaders and some don't is complicated!
Here's a good photo I took. Is it positive or negative looking at it's location? Truth be known it is negative. Another chaser was several km's past me and took a full framed image of this storm. It produced two large 'dog legs' from the negative pool in the middle of the storm, but these punched through the outflow flanking region of the storm. So, in my eyes this would be a positive strike because as far as I can see it came from that lower region right? Wrong!