Risen Martyr Connotations? (DnD 3.5e)

[Berymm]

As a new 3.5e player, and relatively new RPing gamer in general, creating a character that I'm happy with regarding both RP and practicality has been a little confusing and difficult.

My current hurdle is about the rules of the Risen Martyr, and whether it is a good or bad idea to potentially convene with the DM to arrange a Paladin(+/-)10 / Risen Martyr 10 character. My idea for a holy warrior promoting all that is good is kind of coming together, after I managed to be gifted a Pegasus from its father as a companion/mount at level 2 via a series of critical rolls, and already being seen as a minor legend in the monastery and community. But anyway, my questions are:

1. What does being Deathless really mean? Do I gain all modifiers of a deathless (immunities, buffs, ability to not breath eat or sleep, and the huge risk of being insta-gibbed by Evil Clerics)?

2. What does it mean to be a Celestial (Archon). This is basically my first question, do I gain all the buffs and weaknesses described in the monster's manual?

3. Is the 'level 11' final ascension a non negotiable death? Meaning that being a Risen Martyr isn't necessarily a second chance, but more just running on borrowed time, with an identified end date. Or is there a way to continue my adventure as an Archon from the Upper planes, transported to the over world still assisting the party in their adventures?

3.1. If I complete my holy quest, am I also ascended?

4. Also, how on earth do I learn Celestial as a language?

Edit - 5. What does it mean to not have a Constitution score?

Fin.