It's sad that you are giving up. I've used Moodle since 2005, back when things were really slow and confusing. And even then, it wasn't that bad (substantially better than what we had before). You're right, you can't cut and paste to the grade book. But you can upload grades with a csv file, which is a better way anyway, because it would reduce error potential. And yes, when you manually add grades, you have to save BEFORE you move to the next page.
And if your site is so slow that page loads take that long, you need to talk to your tech support about performance. That's not normal. My site is very fast. At my old job, we were running 2.8 and had 18,000 student accounts with over 10,000 courses (five years of archive) and we rarely had any slowness at all and we had a crap ton of resource hogging sql reports running all. the. time.
As for assignment...when you built the module it asked you what date you wanted it due on. If you didn't tell it the right date, you can't expect it to show the right due date. It kind of sounds like your rushing is undermining your production. Look at activity settings, don't assume.
I frequently yell at my computer for doing what I told it, not what I wanted...of course, it did what I told it! Figure out what you actually want and ask it for that.