Here's a better photo of the extra tree... Here is a link that might be useful: Branches growing from Palo Verde root

Basically, it's a waste of time. You won't get enough nitrogen to be worth the energy it takes to collect the seeds and scarify them for sprouting. If you just scatter them they won't sprout for years. What your vegetables need is planting on the Phoenix AZ planting calendar - not the seed company's - drip irrigation, and thick mulch (6 inches of shredded palo verde and mesquite branches) to help retain soil moisture. I took my dirt patch from digging holes with a pickaxe to being able to dig anywhere in the bed with a trowel with just thick mulch, semi-finished compost, and drip irrigation. If you want to improve the soil, plant okra. It loves the heat, has deep spreading roots, and it's edible. After the plants stop producing, cut them off at ground level and leave the roots to rot. You can even use okra to shade other vegetables, such as chilis, that prefer afternoon shade. Plant the okra along the south and west of the bed and the less sun loving plants on the NW side of the okra. Try the AZ forum for help with our specific arid alkaline dirt problems and hot growing conditions. Here is a link that might be useful: Maricopa County veggie guide

Confused? Did the garlic sprout before you planted? If they did probably had something to do with it. Also, where did you get your bulbs? If not local, could have problem acclimating. Should not try to sprout garlic before planting, also they do not transplant well. Should not break apart bulbs till a day or two before planting. Why did they developed small bulbs? Could be several reasons. Stress, not keeping in fridge long enough, planting a chilled bulb into a warm ground, too much nitrogen. Could be a combination. I plant farther south than you, I do ok & I don't put in fridge. Burgundy ok choice but takes several years of planting to get a decent bulb. Siberian iffy, take it from me just because top growth doesn't mean good bulb. Bulb should not develop till spring. Cannot loose anything replanting but you will probably not be successful. Hoped that helped.

Eventually, the saguaro will suck the life out of the palo verde, the palo verde will die, and the saguaro will continue on. If I were in your situation, I'd trim the pv as I felt necessary (or let the saguaro run up through the branches) and leave it at that. At 15', the saguaro is a bit too tall to transplant safely and effectively. If the closeness of the two together look "weird" in your landscape, you can try planting some clusters of small statured cacti underneath the pv (hedgehogs and pincushions perhaps) to increase the cluster and clutter look (and the blooms of both are incredible!).