1. As a MP Pro user, if I move my library onto an EHD, what's the best way to manage any photos that happen to sync with iPhoto when I am say travelling etc (iPhone or iPad plugged into MBP)? How to get them back into my master library file?
The only way to move all the versions, edit history, metadata between Libraries is with iPhoto Library Manager
If you've not edited, then simply exporting from the internal library and then importing to the external one won't lose any data.
2. Once I have my new library on my EHD, what's the max safe size of a single library? Do you recommend running multiple libraries - say one for each year or capped at 100GB a piece etc?
I don't recommend ruuning multiple libraries. They are inconvenient and usually unnecessary. The amount of disk space a Library uses is of no consequence to iPhoto - as long as there's enough space of course. What matters to iPhoto is the number of items. The latest version of iPhoto is good for 1,000,000 items.
3. Following on from Q2, if I then plug this EHD into the back of my new Time Capsule so I can access wirelessly, can Time Machine still backup to that drive plugged into the back of the TC?
About wireless access:
A strong warning: If you're trying to edit the Library (that is, make albums, move photos around, keyword, make books or slideshows etc.) or edit individual photos in it via Wireless be very careful. Dropouts are a common fact of wireless networking, and should one occur while the app is writing to the database then your Library will be damaged. Simply, I would not do this with my Libraries.
Further: See this article
http://support.apple.com/kb/TS5168
Note the comment:
“Additionally, storing the iPhoto library on a network rather than locally on your computer can also lead to poor performance or data loss.”
Tha said, you can set Time Machine to back up external disks.