Nothing to worry about, players will always move on my guess a few may end up at Watford. Those leaving will just be bettering themselves with a step up the pyramid and you can't blame them for taking the chance whilst they can.
I have every faith plans will already be in place to replace these players, the club will have known their intentions internally for some time. Trust the process and the good people involved, nothing to see here. Not forgetting we had a very successfully Dev squad so a few may step up to supplement what's left and new signings. Carly Johns is a perfect example of that, came through the ranks and made her first team debut at 16, in the space of 2/3 years she's grown into a very good player so for all those current Dev squad members it must be a eye opener for what they can themselves achieve.
The majority of teams outside of the top two divisions are part-time so the players either do a 9-5 job or are in full-time education (or a combo of both). Watford last season having been relegated from the Championship remained full-time which obviously gave them a major advantage, I have a feeling Ipswich may also have been full-time too.
Not sure about contract lengths but imagine most won't be more than a year, there's others on here that may have a better idea of that level of detail. I would guess full-time contracts would be more in line with the men's game and how contracts work.