3 weeks , Cancun to Mexico City

Hi Folks,

OK, so my partner and I were able to get a 3 week vacation at the same time, and so have booked flights with Delta from London, through Atlanta to Cancun, returning Mexico City, through Atlanta to London. Giving us 20 days to make the journey & enjoy ourselves along the way. We head out on Sunday 15th April.

Rough plan so far is along the lines of:

Arrive into Cancun airport ~ 7pm, head to a hotel on the coast for a few days
Travel down the coast as far as tulum
Get some scuba diving in along the way
head overland toward Mexico city, via various archaeological sites, interesting towns, etc.
Arrive in mexico city 2 or 3 days before flying home.

So, a few questions, for those in the know:

The obvious first night drop would seem to be either downtown cancun, or the hotel zone. As the first few days are likely to be jetlag recovery and not much activity, a spot on the beach would seem sensible. Is Cancun the right place? or should we head down the coast right away after landing? PDC or similar?
Is the itinery "Doable" given the timeframe available? realistically, it feels like our first week is going to be on the coast before heading inland, is a fortnight sufficient for the rest of the trip? Any obvious gotchas?
Any "must-see's" along the way other than the obvious (am thumbing through The LP at the moment, and reading the board here when I can. Obviously already wishing for 6+ weeks, i can dream...).
Any diving site suggestions? I'm a keen, fairly experienced diver in UK waters, the misses is qualified, but less keen on murkey water back home (she doesnt like reduced viz), so beautiful, clear water, sites at <18m would be the best bet. Scuba outfits to look for / steer clear of?

Also, I've a slight concern on the trip:

We depart the UK on what I think is the first flight out of the UK, landing into Atlanta 1hr 40mins prior to the last Delta flight out to Cancun in the evening. I understand that, for some odd reason, I need to go through immigration into the US despite just transiting through? Given the high security these days (had a lovely long queue at JFK recently...) am I in danger of missing the connection? or is this something Delta et. al do often enough that they have it down to a tee?

Anyone done this run recently, any good/bad experiences? suggestions as to what to do if we do end up stranded in Atlanta for the night?

Thanks muchly!

Gavin.