If you've dropped her off 3-4 times and not stayed, and they didn't ask you to change this pattern for whatever reason, then your protocol was fine, except:
If you've dropped her off 3-4 times and not offered to reciprocate, you probably should have.
That being said, with this still in the stage of you all feeling each other out/getting to know each other, I am pretty turned off by the other mom asking for a play date and THEN inviting herself/son over. I had this happen once (well, just the kid part) where a mom mentioned a play date and, since she was the initiator, I assumed she meant either on neutral ground (playground, museum) or at her place. And then she was all "Okay I'll drop him off at X time" and my house was NOT ready for a visitor! So I had to stammer and make some excuse but I was pretty turned off. Later I learned she was kind of a user (I drove her kid home after an after-school activity many more times than she did mine, for example). So I bet she had somewhere to go and was looking for free childcare. But anyway, that's my issue and it's now about a decade or more in the past!
At any rate, she'd have been more polite if she had said "hey, I'll be in your area on X day--would it be okay if Boy and I stopped by for a quick playdate before we continued on home?" and then you would know that the parameters of her suggested plan were for the beginning.
As far as a parent hanging out, I don't think that's that strange, because she did explain the reason. But next time you offer--and you should, just in my opinion, or stop having playdates at the boy's house as frequently--be prepared that for the first time at least, she'll come over too.
And my sympathy re the distance thing, because my son's entire school career was in three different private schools ranging from a half to one hour from our home (one way), and they *could* draw from areas just as far in the other direction. Ack! So you do have to be proactive in planning these things, considering that factor.