I am sorry to hear this, it sounds crap. You asked What would you do? I would:
Take these two issues as separate, if possible
Your colleague was mean to you but clearly he was upset by this too.
Have you made your peace with him? If not can you have a chat over coffee or by phone and clear the air.
What caused him to be upset with you?
Is it personal, friendship, romance (or unrequited love on his part/attraction)?
Or was it professional (you did not do a good job on something or he didn't and you called him out)?
Or was it totally separate - he was jealous of your work/promotion blah blah etc?
Once you have made your peace with your colleague, I would forgive him and those he texted but treat them with caution. Maybe he wanted you to see the text, maybe he wanted it come out. Try and get to the bottom of it is you wish to BUT in future I would keep work and home life separate.
Do you share much about home at work or engage in chat about others home lives etc?
If so, I would suggest do not share too much about personal/home/health issues etc.
Make sure you have plenty of great people to do that with away from work.
It is not necessarily bad to share with people at work and sometimes very positive to do so. My colleagues know a lot about me and I know a lot about them. However, my work place is a very friendly work place and I know other teams may not be so close.
I just feel if things go wrong, work gets very hard, mistakes are made etc, if there is too much 'home talk' at work some may claim that that causes disruption etc.
In terms of your manager I would either write an email and express my unhappiness about the very unprofessional 'mob-handed' behaviour of my manager and assistant - and send it now; or write email but keep and see how I felt before my return.
EG
"Then the manager comes into the room as well, closely followed by the assistant manager. They say they have been concerned about me but have not felt able to ask if I'm OK, but it seems it's okay for my colleagues to message each other about me."
This is so unprofessional, they should have asked for a meeting and been much more formal and helpful. If they had concerns they should be able to name them.
I'd also gather examples of praise of my work and good reports/references by email that I had already had from colleagues and if there are any discussions on performance I would produce a sheet of printed emails saying what a good job I was doing.
If you have left work for your holiday just make a hand written list of examples you remember. You can check details later.
The issue is their issue but it is affecting you. Maybe compiling evidence of their failure and your good work will prepare you for return.
And then chill. Be nice to you, do nice fun things, build your confidence up.
Pamper yourself. BE nice to YOU.
Good luck. 