All of the remotes you mention would be fine. They can all have up to 255 activities, the only difference is the number of activities displayed per screen....activities and commands can be displayed over multiple screens if required.

What you pick depends mainly on your budget, and what you like the shape/layout of the best. I also believe you are correct that the HK tech support is not up to the same standard as Harmony, but you will get plenty of help from the Harmony forums here if required.

The 550 is actually one of Harmonys newer models, and is just as capable as any of their other models...the only thing it's missing is the colour screen and rechargeable battery.

All of the Harmony remotes are at least as capable, and in my opinion far more capable than an MX700. You need to look at an MX950 to be able to match a Harmony. I used to like the MX remotes for certain purposes, but these days I could never recommend them due to their new customer un-friendly poilicy.