About your Fremontodendrons getting tall and falling over down hill, yes they do. When mine get to about four inches trunk diameter or more, and the soil is saturated as it is now, they just lay on the ground down hill. I cut mine off at about four feet from the ground, attach a "come along" and pull back into vertical. I lash it to a "t" post to keep it vertical for a year or so, then remove the post. The tree recovers and puts out a lot of lateral branches. I don't think I would have this problem if the tree was in a full sun location, where it would tend to grow straight up. Al

I have some San Gabriels here for many years. We are about 300 feet above Calistoga. Cold has never been a problem. If you get through the first two years the only problem will be keeping them from over growing their space. One I had to remove because it grew over an access path. If planted where another tree will shade it from the sun it will grow at a very large angle. One over 20 feet tall on a slope grew at such an angle it pulled the roots out of the ground during the winter when the soil was wet. I cut the trunk off at about four feet, it was 6 inches in diameter. I drove a steel fence post in the ground up hill and with a come along attached over a period of days was able to pull the trunk close to vertical. With it lashed to the fence post anchor it recovered and now I keep pruning it to prevent another problem. Al

So, you know it is against the law for others to mail plants/plant parts/cuttings/bulbs into California? We have agricultural restrictions (thus the Ag stops coming in and out of state for cars). Individual choice can have disastrous affects on wild lands and the ecology. Did you know that Citrus Greening - the disease that has almost wiped out Florida citrus crops and is now in California was most likely brought into CA illegally by a backyard gardener? http://ipm.ucanr.edu/PMG/PESTNOTES/pn74155.html  And, to Florida through illegally imported plant material? https://www.invasivespeciesinfo.gov/microbes/citrusgreen.shtml  Other states have Japanese beetles that feed on about 300 species of plants, flowers, and fruit; squash vine borers that attack vine crops of summer squash, winter squash, and pumpkins (I know gardeners that can’t get any summer squash at all anymore without a great deal of extra work, most just give up) and Tomato yellow leaf curl where it is not uncommon for losses of 100% to be experienced in fields with heavily infected plants. The host plant can be very different than the ones we see infected. There is solid reasoning behind our laws and quarantines that no plant material (living, dried, bulb, tuber, rooted cuttings, cuttings) may be shipped in to California unless it is from a company with phytosanitary certificates. https://www.cdfa.ca.gov/plant/PE/transport_animals_plants.html "The importation or smuggling into California of live plants and plant product... will have serious legal and financial consequences for the persons involved". "We are free from many invasive insect, weed and disease species that wreck havoc on agriculture and environments in other states and countries. Our State is surrounded by natural barriers — towering mountains to the north and east, scorching desert to the south, and vast ocean to the west. Most plant pests cannot cross these barriers on their own; however, the State is under constant threat of pest introductions by man". Do No Harm. Please continue to spread the word that trading out of state plants and cuttings into CA or buying them from individuals on Amazon, eBay or Etsy is illegal and a really really bad idea. (quotes from https://www.cdfa.ca.gov/plant/pe/ExteriorExclusion/borders.html ).

Yes, I've read that people usually kill these with too much kindness, or even a little. People generally curse the soil in my valley since it seems like it is about 30% rocks and 50% sand. But it is perfect for many natives and S. African and Australian imports.