This thread is right up my alley with my T-squares! Just a preface for those who don't know (I know you do but to anyone else reading):
ASC 6.34 Libra / BML 9 Libra +
Jupiter 2 Cancer / MC 8 Cancer / Chiron 13 Cancer +
Uranus 1 Cap / Saturn 7 Cap / IC 8 Cap / Neptune 9-10 Cap / Moon 15 Cap
Oh and Vertex @ 15 Aries + of course Antivertex 15 Libra
Then also the most important one:
Venus & Juno 25 Virgo
square Uranus and Jupiter (out of sign, but well within orb)
1. Please talk about your experiences about having a person's planet turn your t-square into a grand cross? Was it good, bad, life-changing, extra intense?
Yes, especially until about 2 years ago, I was literally looking for and yearning for my missing leg. Lots of interpretations link it to a table with only 3 legs and I think that's exactly it. It's okay, but you know you'd be a lot more stable and 'complete' if you had that fourth one. There's always a bit of stress especially for the apex planet (in my case Venus/Juno and ASC).
And when I lost touch with/fought with/broke up with the missing leg person, it was like a big part of me was ripped away; a source of stability suddenly breaking down under my feet.
It gets a lot healthier acting out the missing sign and house itself though. As I've said for years on this board, I especially get the Aries/Mars influence from female friends. And with my Mars-Mercury conjunction, at least the way I think and communicate is straightforward, so seriously, what was I thinking?
But I think that's key to attracting the healthier expressions of your missing sign. Remember that it is still your chart - it's you, somewhere deep down. (Could be my DSC being in the mix though, it was all relationship stuff.)
2. In particular, do people whose personal planets lock into your t-square seem to play a more significant role in your life than those who don't?
Yes, they do. Now, it's not a hard-and-fast rule because my dad also played a major part and he doesn't complete any of my T-squares (though I do his I believe, plus Vertex and Node contacts), but if I'm going to think of the most memorable people, the majority that pops up into my head right away, they did. Yeah. For good and bad reasons. Some of my most important female friends did it - many of the past and some in the present. They inspire me.
3. Essentially, what does a synastric grand cross really feel like? Do you think relationships have had a stronger effect on you than they have on others, who don't get into grand cross muddles as often...And do you seem to be more obsessive or fixated on certain "grand cross memories"?
I think so, yes, but again, with all of my angles involved especially the 7th house, then Venus and Juno in another, I can't really tell if it's the pattern or just these influences. I am also strong in Saturn with a strong dose of Cancer, plus Sun sq Pluto; I tend to hold onto the past.
While I don't think relationships of people without T-squares are less meaningful or deep, I do think they're different. The T-square just propells you into such a specific direction, it's like you have to go there or else, subconsciously, you will keep having certain problems. Like in my case in my social life, relationships and the past (IC). It feels like the NN, but more compelling, like your "comfort zone" will leave something up to desire.
For all 3 questions, I've noticed I had a stronger reaction to people who put planets on 25 Gem/Sag where it would be the "real" square instead of that out of sign one. And then they of course had a Pisces planet around that degree.
And on another note, I think the apex planet stands out as strongly as if it were on an angle or a singleton. I mean, if it gets that much stress and holds an opposition together? Maybe not to others, but to you, it can easily become central to your life.
Hope this gives enough food for thought.