Question 4 is the most borderline, I think. So I'll address that one: If question 4 is about mechanical tools that are only ever used for gardening or landscaping (e.g. lawnmowers, hedge trimmers), then there is a very high probability that gardeners and landscapers who use this site will be capable of answering and interested in the answers. And gardeners/landscapers might even provide a more relevant answer than DIYers or mechanics, since they have more experience with the tools in question.
I for one, don't care a single bit about DIY, Home Improvement, or Mechanics. I'll probably never ask a question on those sites, and certainly never answer one. It's unlikely that Google would even ever bring me there, because that's what I pay my contractor (or ask my dad) to do for me. I didn't previously know there were Stack Exchange sites for those topics, and I don't really care. But if someone asked a question about how to build raised beds or how to clean out their lawnmower's carburetor, I might be able to answer them because I have experience with those things and they are directly linked to and in support of gardening and landscaping (albeit borderline).
I don't think that overlap of topics is the worst thing in the world. There's certainly a huge overlap between Server Fault, Linux/Unix, and the Ubuntu sites, far more so than between landscaping and DIY. I assume that all three sites exist because there are active communities for each who see them as separate, even if overlapping.
That said, perhaps this site should be renamed "Gardening and Horticulture", since many of the biggest contributors would rather draw the lines to exclude most non-plant related landscaping questions. Perhaps landscaping needs its own site (even though that site would have a lot of overlap with DIY)? That would certainly make the "on-topic" dilemma a whole heck of a lot easier.