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Hi Mariano A)As you mentioned Google supports up to 50,000 URLs in a Sitemap, This limit applies only to the number of URLs referenced specifically in <loc>tags.</loc> URLs referenced as hreflang don't count towards the 50k URLs. B) Yes try to implement the sitemaps as they are different domains.

About B, and maybe its related to If we create a separate sitemap, we wouldn't be "saying to Google" that www.domain.com/a its the same page, but different version as m.domain.com/a. Using alternate media you are telling google that it is the alternate version, not a completely different page; something similar to what happens with hreflang and domain.com / domain.co.uk / domain.de, or with canonical urls

The Google Webmaster Central blog advises to use a 503 for planned downtown (and bandwidth-overruns) in their January 2011 post at http://googlewebmastercentral.blogspot.com/2011/01/how-to-deal-with-planned-site-downtime.html.