Some miscellaneous feedback on your route questions - (keeping in mind that my feelings about some of these trails is pretty subjective, in part):
Hobbes wrote:The key (and long) day would be crossing both Glacier & Blue lakes pass to reach the meet-up spot, so I'd want to be camping west of TI lake to get an early morning start.
I'm sure you (being a very fast person!) may be able to pull this off in a day, but do you want to? This stretch is what I call the Magical Kingdom section of the SHR. It doesn't get any better than this. It is something to savor. There are plenty of camping options along this stretch. I should mention also that even for a fast person, this section can be very slow going. Top of North Glacier Pass is long slow talus slog; Catherine to the southernmost Twin Island Lake is steep and very convoluted (typical confusing metamorphic landscape); navigation from there to Blue Lakes is not straightforward.
Starting points: You bring up the idea of starting from Agnew or Devils PP onto the SHR, but you don't mention where exactly you would intend to join the SHR. Lots of options here - listed from south to north:
1. Devils PP to Beck Lakes. I haven't done this, but ask around here about Nancy Pass. (Slow?)
2. Devils PP to Minaret Lake. The stretch from Minaret Lake onward is magnificent, of course. However, I don't find the trail to Minaret Lake to be interesting at all. And that's a long trail approach, longer than what's indicated on most maps. (9 miles?)
3. Agnew up the Shadow Creek Trail to Ediza, then join SHR towards Nydiver Lakes. Wonderful all around. The Shadow Creek Trail is beautiful. Crowded till Ediza.
4. Agnew to River Trail to cutoff up to Garnet Lake. Turn Garnet around its north side, to reach the basin west of Garnet and under Banner Peak. (There is a little lake up there.) Join the SHR there. Personally, if my goal was to reach western end of TI Lake by Night 1, I'd probably go this way. You lose the people faster, and you get to see that great basin west of Garnet, and the bump from there into the TI basin is very easy going.
5. Agnew to River Trail to TI Lake as you mentioned.
As for your exit:
I enjoyed the Isberg Pass Trail (which compromises the first ~5 miles of trail that you'd do). I was there during high season (early August) and we did not see one single other person the whole way to Lewis Creek. The trail is narrow and not heavily trodden. Just how I like it. From Lewis Creek to the JMT it is also not very crowded. This is not a super-heavily-trodden area of Yosemite. Mostly just some folks at Vogelsang.
If you then go southbound: Let's say you need to return to Agnew Meadows. From TI Lake, you could take the "High Trail" (the PCT) instead of the River Trail or the JMT. You'd lose the JMT crowds, and get much more spectacular views than on the River Trail.
Rafferty Creek Trail. Plus: Very fast walk out. Minus: About the least scenic trail in Yosemite. Wide, forested, mule poop, etc. etc.