Sand Volleyball and Indoor Volleyball are considered separate NCAA sports, and thus have a certain ranking within the NCAA hierarchy with regards to scholarships. For instance, NCAA DI women's basketball is ranked higher than DI Volleyball, so if a player participates in both sports and is on a scholarship, they must be carried on the higher ranked sport which is basketball (no basketball players may be carried on a VB scholarship).
Indoor is higher than Sand for volleyball, that is why if your daughter is going to play indoor, she can't be on a sand volleyball scholarship (the school's first offer) and must walk on as an indoor player.
If you have read my site for long, or read Inside College Volleyball, you know I am very leery of any walk on to scholarship scenarios; not because all college coaches are selfish twits, but rather there is so much uncertainty within today's college volleyball environment. Any promise you receive from the coach, who is the representative of the school, is nothing more than words in the air.
Absolutely ask for what was verbally explained to you in writing - If they won't put it in writing, it is not real. Just remember that if that coach bails, or changes her/his mind, then you do not have a legal contract but having it in writing can only help.
Yes, non-athletic scholarship players are allowed to report/train/travel/compete before school begins and the athletic department will have a protocol for paying the expenses for pre-season activities for walk on players up to a specified date (usually when the residence halls/cafeteria opens for the traditional education term).
Good luck and just be hyper aware of what you are being told and not told; and, and, and keep asking questions!