I looked at Blue Star and to June 30 they seem to be bypassing Symi .
Have I read that correctly?
Quite possibly, routes change thoughout the year. Although the timetables for the Tilos-Symi Blue Star route I had in mind (Piraeus-Kalymnos-Kos-Rhodes & Astypalaia-Patmos-Lipsi-Leros-Nisyros--Tilos-Symi--Kastellorizo) have only been posted up until 31 March so I am not sure where what exactly you are reading.
To be confident you have to go to www.openseas.gr and backtrack to May 2013. If you can find ferry service every day in May 2013 then it is reasonably safe to assume that it will be the same in May 2014.Edited: 09 January 2014, 05:44
Direct service from Tilos to Symi and back is a government-subsidised "lifeline" route so you can reasonably rely on it operating in May. However there is only meant to be one sailing a week (currently Fridays, operated by Blue Star and likely to remain the same all year).
Rhodes to Symi (and back) has for many years been at least daily between Easter and the end of October.
Kos to Symi had about 5-6 sailings per week last year, but rather more in the opposite direction.
Because of this you may find Tilos-Rhodes-Symi a better bet than Tilos-Kos-Symi
However timetables for the periods either side of high season are rarely published more than 6 weeks in advance, and a bit of flexibility is needed in planning any itineraries.
We'll assume Rhodes to Symi will be running in May . Worst case then would be to have to backtrack to Rhodes again - but a good chance of ferrying on to other Dodecanese islands and then back to Athens again. We are well familiar with Turkey and would rather make our way back to Athens via Cycladic islands we've yet to explore, like Amorgos ,Folegandros and Milos ....if we can find a route by ferry or plane.Edited: 09 January 2014, 11:44
Definitely no problems on the Rhodes-Symi-Kos axis which manages 5 sailings a week (weather permitting) even in January. It's the Rhodes-Symi section that ramps up from April onwards to 2 or 3 ferries a day.
Amorgos ought to be possible without backtracking as there are Kalymnos-Astypalea (Nissos Kalymnos) and Kos-Astypalea (Blue Star) services, some already bookable for May, as are Astypalea-Amorgos ferries, also Blue Star.
There's also a Rhodes-Kos-Santorini-Piraeus route from April onwards which might provide a way of getting across to Milos via Santorini.
Dodekanisos Seaways are now bookable to end September, and Blue Star to end May, service very similar to last year's