If you don't trust your boyfriend that is a problem in your relationship. That is not his ex's problem or responsibility and the fact that you would have him block her and that you are currently posting asking if you should message her does make you sound incredibly young and immature (probably why she said that about you)?
Has your boyfriend ever done anything to give you cause not to trust him? If he has then that is something you either try to work out between the two of you or you end the relationship. Based on the above it doesn't sound like he has done anything.
If it is simply that you don't trust him because you were betrayed by someone else-- not him-- in your past you need to work on your trust issues. It isn't fair to become controlling or blame others for your issues. If you can't figure out how to fix your issues so that they do not result in problematic and unhealthy relationship behaviors it is going to eventually destroy your relationship. That's what everyone here is trying to tell you. They are coming at this from a place of concern for your well being because you're on the road to becoming controlling in a way that indicates psychological/ emotional abuse. Listen and stop before you become that person. You are wildly out of line and basically admit it is due to your own insecurities. Fix those insecurities now.
The reality is most people in healthy relationships fully trust their partners. It sounds like you don't and don't have good reason not to trust him (if you had good reason the solution would still be to end it not to try to control him). So figure out, perhaps with a counselor, why it is that you can't trust a partner who has never given you reason not to trust him and how to address that fault in yourself. Stop blaming her. If you figure out that you're not in a place to be able to trust a partner, well, maybe you aren't ready to have an adult relationship.
In a healthy relationship another woman could totally throw herself at your partner and you would and could rely on him to set boundaries without your snooping or making him delete someone. It sounds like she has been appropriate and even if she wasn't being appropriate you have no reason not to think he would handle it. Messaging her telling him that he was the one who chose to set boundaries was above and beyond the call of good boyfriend behavior as is showing you messages, checking in with you and deciding not to attend the wedding (mainly, it sounds like, simply for your comfort). You are acting like the crazy, controlling, immature girlfriend and messaging her is just going to confirm all the problems in your relationship to her (plus is just rude). Take this as an opportunity to get yourself under control.
Honestly I have good relationships with most of my ex's. If someone's ex messaged me anything like what you're proposing I'd probably express some concerns about that ex and his well-being as a friend and then distance myself from both of them. Because no one wants to see their friend with a controlling partner and very few people want to stay friends with someone who thinks the behavior you are exhibiting is appropriate or healthy.