That is when we've gone the last couple years!
1. Is it packed this time of year? Yes. So much YES. Don't trust crowd calendars. It WILL be crowded every single day.
Disneyland runs full operating hours around Memorial traditionally so that's what they think of the crowd levels. Go in knowing what you want to do, use the fast pass system, make dining reservations in advance & if the crowds are too much for you take a break in downtown Disney, your hotel, or hop to the other park. The best time is first thing in the morning. The first couple hours are the least crowded.
2. Grad Nite. Yes you will notice them if you are in the right park at the right time. They usually hit up DL since the party is in DCA. We notice them around lunchtime, but there aren't enough to affect the park "atmosphere" for us. By dinner DL is thick with grads continuing to firework time (TIP: do NOT see night parade and firework show on grad nite. Although the late parade on grad nite was fine for us). In our experience there were still tons of grads in DL by 11pm FYI. They mainly affect "big rides" with regards to crowd levels- Indy, Space. As for the park "atmosphere" it's luck of the draw of your grads. Some are behave fine, others would make a sailor blush. If you are seriously concerned about rowdy grad nite Seniors stick to DCA from dinner forward on grad nites. For more grad nite info search "grad nite" here on the DL forum. We've had many a discussion!
3. May 30th & weekdays in June. Summer really kicks off on Memorial Day here in CA. Some schools are out (but not the big districts like LA) & Memorial kicks off summer in Disneyland. We did memorial and the weekdays following a couple times. Memorial is noticeably busier than the rest of the week (which is still quite busy). TIP: if you are driving arrive early, we stay onsite now but one year we drove and were cycled through Disney traffic for an hour! And my understanding is the parking situation has only gotten more unpredictable over the years. Better to be at DL an hour early than an hour late! At least in my book
Our last trip was the week before Memorial and I preferred that week. Hours weren't quite maxed (DL closed at 11pm) but our hotel was a lot quieter than our previous post-Memorial trips, making for a more relaxing afternoon break when you aren't hovering over some poor soul waiting them to vacate their pool lounger

As for in park crowds I don't recall much of a difference. Your dates should all be very similar crowd wise except for Memorial, that will definitely be busier. (Memorial is an actual day off in CA and the US, it is a holiday of remembrance and honoring those who died for our country. So to some people it is a very important day. What it does in the general cultural fabric of American society though is kick off summer backyard BBQ season and school vacations.)