I had asked Arcangelo who donated the bulbs about cultivation.
I asked if it was ok to share and he said please do.
First, I hope I can offer you some helpful information, but my greatest asset in growing Gethyllis is California. Except for my seedling pots, which is what I donated to the BX, I grow my Gethyllis in the ground. I have made sand and rock beds and this is where the Gethyllis are. In the last month, our nights have started being in the low 50s, with some rain. This is when the leaves emerge. In fact I was a little concerned that I was sending the bulbs when they had started leafing out. This is also the time that fruit emerge, see photo. I will be sending seeds to Lisa for the recalcitrant seed exchange. We can get rain from November to the end of March, tapering off with possible rain into May. Leaves will die off by mid to late May. Flowers usually emerge in the middle of June. Most of my plants are in full sun, but some are in partial morning shade. It definitely makes a difference for flowering for them to be in full sun. Where they are planted in full sun, they usually have a "nurse rock" to keep cool. Leaf spiraling in G linearis(?) is best in full sun. Summer temps are usually 80/65, with heat waves in the 90s being common. Of course the heat is not a problem since they are dormant. Some of the plants had been planted near drip irrigation running every five days, but in sand. No negative effect in my opinion. The G ciliaris ssp longituba seem to be able to handle a bit of summer water. One of my youngest plants is G verticillata from 2014 seed. It has been in the ground for two or three years. It just emerged last week and has split itself into two bulbs. That is exciting.
Now to the plants grown in pots. Remember I said that I am about to donate seeds. They are easy from seed and very tough. I plant them in either pure sand or a blend that has at least 50% sand, pumice, and my new favorite minor elements decomposed granite and 3/8" granite. The decomposed granite in small quantities holds moisture better than sand, but can hold too much in large quantities. Pumice should not be more that 30%. They can tolerate some organic matter and some regular dirt or compost in that mix, but I generally haven't included any. From time to time I do a light feed, but I confess these Gethyllis were grown more because I had them than because I wanted more. I am not generally after bigger better and prefer growing things lean than to maximum production. But it is nice to hear that they are desired so I will keep growing to share them. Seeds are sown in 3.5" x 5.5" deep pots that fit in 10-20 trays. I get them from Greenhouse Megastore. I then move them up after a year to 4" deep pots that are 9" deep. The deep pot is of utmost importance since the bulbs go very deep. I have a rack that hold these tall pots together so they don't have much surface area exposed to heat. I can't remember if I have used a sand plunge since they are dormant by the time hot weather comes, but I would recommend this for the ability to keep moisture accessible to the roots without over wetting the bulbs. I just use a white five gallon bucket with holes in the bottom filled with sand. Once the leaves die down in late spring, I put the pots in the shade under a deck and ignore them. I do not water the pots again until early September. They get one thorough wetting and then wait until rain (or I start to water in November if it hasn't started raining). I forgot to include instructions that they should be planted very deep. The tops of the tunics should be at ground level or just covered, but they should pull themselves down. I have never had any pest problems. I don't think we have the famous Narcissus bulb fly here, but I don't know how they will be received in other areas. My winter temperature averages are generally low 50s in the day, low 40s is average at night. In January and February we can have many nights in the 30s, and a few times a year it will go to 28.
So to answer your three questions more directly:
High light level.
For the substrate in a pot, at least 1/2 to 2/3 sand. Pumice, decomposed granite, 3/8 or 1/4 granite and a bit of compost are other options. In a bed, the sand could be even more or all.
Water once in early September, even if it is hot. Then water from November to early May or until the leaves wither. I would recommend the sand and pumice mix because it allows for watering that never will be too wet.
Let me know if you have follow up questions, also feel free to share this to the list or let me know if you recommend I do that. I am not on the forum.