Question about the expansions
This support FAQ should cover all of those questions.
https://help.guildwars2.com/hc/en-us/articles/115010688287-Guild-Wars-2-Path-of-Fire-FAQ
In short.
There is an option to buy PoF with HoT as a package deal that allows you to add HoT to your purchase of PoF for $20 more.
It is not mandatory to have HoT before PoF.
(edited by JustTrogdor.7892)
You can get both expansions right now, begin playing in HoT now and play in PoF in about a month. The basic PoF is $30 and if you get HoT at the same time, that is another $20 (normally $30).
You can play PoF without HoT. But why miss out on gliding!?
No, but cheaper (at least right now).
1. Already offered at buy.guildwars2.com
2. No, see #1.
3. No, you can purchase them separately, or as a bundle, in any order you choose.
Good luck.