Non-ASCII characters are indeed messed up. I wrote the HTML parser about 15 years ago. Back then my programming style was so bad, that I'm afraid nothing short of a complete rewrite of the parser will be able to fix it.

The Archive.org crawl is way too old to be of any use. And the Common Crawl corpus is only marginally bigger than what I crawl myself. There are currently just over a billion pages in my search-index and I can recrawl those about every 45 days. Crawling right now at about 650 URLs/s and 200mbit/s. Only about 60% of URLs crawled end up in the index. The rest are errors, timeouts, redirections, etc.

I have been thinking about adding an API. Rate-limiting is a must as you have correctly said. I'm already in a constant fight against SEOs who try to scrape the search-results and who apparently don't see anything wrong in making 10+ queries/second.

The API would definitly not be a free-for-all, but sort of like "send me an email and describe what you want to do with it, and if I like it I'll give you an API-key".Together with a primer on what not to do of course. :) If people do more than a certain amount of queries/month, then it would also have to be paid.