Hi Felicity.
I'm sorry to hear your moving experience hasn't been as pleasant as you hoped. It sounds like your issues are not with the move itself, but the merging of two sites. Please let me know if I am mistaken.
Issue #1 - Page A outranking Page B. You want to evaluate your entire site and determine which pages you wish to rank for each relevant search term. If you have a page from the blog ranking for "Garden Art" but you also have a page on your main site ranking for the same term, you NEED to make a decision, or else Google will make it for you.
It sounds like you prefer the page on your main site to rank for garden art. That's great. The 301 you added works, but it is far from an ideal solution. First of all, your users now have to be 301'd from your old Garden Art blog page to your site, then another from the blog page to your main page. You are leaking link juice on the double hop. IF you were to keep this solution, then go back to your old blog site and 301 the category directly to your main Garden Art page.
The better solution is to optimize your blog for another keyword phrase. For this example I will use Garden Decoration as the new phrase. Replace all of the "Garden Art" phrases in the title, description and page with "Garden Decoration", except for maybe 1 or 2 instances. In those cases, use anchor text on "Garden Art" to direct users from your blog to your website page.
This process allows you to keep all of your existing content and sends a clear message to Google that your main website page should rank for Garden Art, not your blog page.
Issue #2 - You are missing meta tags on 500 pages. Which meta tags? Description? Title? Every page should have unique meta title tags and meta description tags. If it is another meta tag please let me know which one.
Issue #3 - Is it ok to leave the long URLs? You could do so, but I would fix them. I run into these situations all the time. Do I leave something alone, or do I commit time and effort to fixing a problem to the way things should be. I usually vote for the latter unless I know there will be a better opportunity within the next year (i.e. site redesign) in which case I will plan to make the change then.
The bottom line, you merged two sites and made some mistakes. Fix all the mistakes. Make the changes using the best known practices, and then most of your traffic should be restored. IF the change was a good one for users, then you can even see an increase in traffic.