Good to see you are enjoying punBB.
It's fantastic actually, really easy to follow the code and alter it to do what I want rather than being stuck with what someone else thinks is the best way of doing things. Gotta love the speed too 
Does you mod on line 162 work ? What does it show exactly?
It displays the last post summary on the forums index page taking into account any sub forums that exist. ie:
Was the default look after I installed the mod originally. The forum Xennate has a sub forum with 2 topics posted, the post count is correct but there is no last post summary, it is left blank.
This is what it looks like after the tweak detailed in the last post. The post count is still correct but now you also have the last post summary like the rest of the forums that have no sub forum.
Stir is the sub forum in Xennate as you can see from this shot.
Your's will no doubt look different asthetically since I've been hacking away at various elements to change the way the punbb looks to some extent, thats nothing special though mearly just css and some template/php changes.
Suppose I should add that these forums are for a mud hence the odd forum names 