Just get used to being 'the bad guy' for a while, unfortunately you got automatic membership when you birthed your child.
My mum had a different issue (still linked to mh) where she arrived with armfuls of 'stuff'. Some would be cooked food, newspaper clippings, lemons from her neighbours tree, some random mail-order catalogue, a weird abstract 'gift' for dd, a list of topics she wanted to talk about. Nothing wrong with this intrinsically, but she'd barge in with no greeting (and no response to ours) and launch into an 'urgent' 50 minute monologue on it all. Bloody exhausting, and quite sad when little dd sat expectant and confused that nonna was ignoring her.
I came to the point you're at. The big NO. For 6 months I literally opened the door by stepping out and pulling it closed behind me. I'd take the bag(s), put them down. I'd hold her hands, look her in the eye and wait. Then I'd say 'hello', and kiss her cheek. If she didn't say it back (but started babbling on), I'd close my eyes and wait. Wash, and repeat. Sometimes this panto took 15 minutes.
I had to ignore the 'looking like an idiot' part, ignore the 'wishing she'd be normal' thing, etc. I stuck with it for what felt like forever. I still have to do it occasionally (she's a 'give an inch, take a mile' type). In general, she got the message. I wasn't talking at her, making it clear she had to change and why what she was doing was wrong, bad, abnormal, etc. I just set a boundary about it happening in my house.
If I were you, I'd create a magic imaginary barrier against 'stuff' that circles your house (and tell her about it in writing). Everything she brings goes into a box (outside). Let her know it'll be delivered back to her house monthly (or on whatever schedule you visit the PILs). She can choose one thing to bring in, you say smiling. If she sneaks more past you to your dc, you just take it out of his hands, outside to the box and wear the tantrums on both sides. No comments, no exceptions. The upset your dc feels at this is nothing compared to the shock of being a spoilt baby in a tough adult world one day, right?
Sometimes you just have to be the bad cop, and patrol your patch without shame or embarassment. It'd be nice if everyone understood and appreciated your (researched and considered) motives, but so what if they don't? You only have your conscience (and your future adult child) to answer too.