Hey Oscar - my second language is French and I couldn't possibly have asked the question in my other language as well as you have here, so don't worry about the language issue at all!
For your question about changing the index.htm to something with a keyword: no, that would have absolutely no effect. As long as the redirect was with a 301 code, the search engines and the visitors will never know that page even exists. That's the whole purpose of the 301-redirect. it makes that page invisible to the search engines.
I would also strongly advise not to switch to a keyword-rich subdomain. that's just asking for a lot of headaches and problems in the future.
What you are suggesting here is trying to create what is called and Exact Match Domain (EMD), meaning the title of your domain matches the keyword you are trying to rank for. The value of these has recently been lowered by Google anyway, and as I say, doing it using a subdomain will cause all kinds of hassles, not to mention confusion for your users.
If I were you, the area i would be focusing most of my efforts would be on building and earning links form other websites. At the moment, you essentially only have one other domain linking to you.
In addition, trying to rank with a very competitive general term like impresión digital will be very difficult for a new site. Better to use terms like impresión digital en Chihuahua and others related to where you offer your services. This is assuming that most of your customers are fairly local, not from all over the Internet.
Hope that answers your question? If not, be sure to let me know.
Paul