Hmm - I see both sides - if it appeared the SM had the children's interests at heart, oppose to appeasing her sense of insecurity, then making arrangements or contacting OP wouldn't be out of the ordinary in some blended family situations.
BUT, as OP has stated and given examples of, her exH who she has co-parented with for 6 years with no major issues is now facing objections not because of OP, or because of exH, but because of SM. That's not working in the interests of the children, that's altering boundaries because of insecurities. SM may very well not have that kind of relationship with her ex, but funnily enough, no two relationships are the same - so her point there is largely irrelevant.
Any decisions about parents evening, school plays or any other co-parent events is up to the parents. If exH chooses off his own bat to go alone or separately, that's his choice and shouldn't be because the SM has dictated.
I'm a SM, and I wouldn't dream of dictating to my DP what he should and shouldn't do with DSS Mum - and this is with a very acrimonious year behind us. I've suggested things, such as not entering the house but to pick up at doorstep, which has been echoed by EVERYBODY else we know, mainly because for a long time, my DP's ex would take any opportunity to get DP on 'her turf' and have a poke at him. After a couple of months of just doorstep pick ups, she stopped the attitude. DP choose off his own back to see teachers separately on parents evening, not because I said so, or because I can go too, but because DP finishes work late and can't make the same appointments as his ex, who always insisted he had to make a 3pm slot.
I do have contact with his ex, because she often prefers to text me arrangements and/or medication needs - DSS has allergies, asthma and bad eczema and with no disrespect to my DP, he isn't really very diligence about cream application or the latest dosage of allergy medication. DP also hates doing homework with DSS as he thinks at 6 years old its ridiculous, so DP's ex knows that speaking to me about it is more likely to yield a result. And I'm cool with that, because from my POV, I love my DSS and want him to grow into a stable, well rounded, healthy boy. If it means having to communicate with her, I'm OK. DP is also pretty useless with organizing things outside of work, so both DP and DP's ex usually run stuff past me which I'm grateful for, as it does impact my life because we have DSS each weekend.
Generally though, after a really hideous year of DP and his ex coming to blows (DP being undermined and provoked a lot of the time, although he was as guilty and took the bait) they now are on good terms, and if DP's ex wants to contact me sometimes, I know how to ensure boundaries are in place now. Before, she'd use opportunities to poke at DP without any response from me, which she disliked as she wanted back up - but I made it quite clear my contact with her was in case she needed to pass info on about my DSS, nothing else. She sometimes oversteps the boundaries and tries to be over friendly but I appreciate that she's thankful I'm in her sons life and I'm glad that for the most part, she gives me the freedom to bring up her son in part with DP.