You don't need to really worry or stress about the missing meta descriptions and long titles.
Meta descriptions do not impact your rankings and Google will automatically create a description for your page if it appears in the SERPs.
Title tags that are too long do not impact your rankings... at least not directly. If your title tag is over by 10 or even 20 characters, it will not impact whether your pages ranks or not. The 70 characters is a suggestion as that was the number of characters that would display in the SERPs; however, now it is based on pixil width. The only other important info you need to know about titles is that you put your most important keywords towards the beginning of the title.
If you are unsure about how or are unable to edit these pages to add or edit the description and title, it isn't going to make our break your site from a ranking standpoint.
Some CMS will automatically generate 301s if you edit a URL's structure. It does this so that any old links pointing to the old URL will be brought to the edited URL. The CMS will not fix broken links that point to the old URL, but on the server side, if someone clicks on an old, broken link, they will be brought to the edited URL page - if that makes sense.
I understand that you want to attack warnings and notices and get things perfect; however, sometimes it just isn't possible. Whether it is a CMS issue or knowing how to fix something complex - what does matter is that you investigate each warning and notice and make sure that it is not negatively impacting your site. From the sounds of it, the handful of warnings and notices you have are just fine.
Hope this helps answer your question.
Mike