First one looks like Gibbaeum dispar. No idea on the others.

1. All markers will fade, even the ones that say they are for garden labels. Its a very easy fix, use a pencil! Never fades, lasts for years.

2. No idea. Even in the mojave desert heat i use clay pots. I only water once a week or sometimes once every 2 weeks.

Yup, sure looks like it.

Pretty sure it should ebony not evony. As in ebony and ivory...
The one with the black tips is ebony.

I agree, Pilosocereus gounellei ssp. zehntneri. Same flowers as Pilosocereus gounellei.

Definetely G. kunzei. Especially along I-10. Near the stateline of CA and AZ they can be found all over, don't even need to stop, you can see them right at the edge of the interstate. But of course, it's no fun just seeing them at 70mph

Great habitat shots!
Echinomastus indeed.

Opuntia robusta

Looks like a Ferocactus. It was grown in too much shade making it stretch (etiolate). The main growing point was damaged somehow. Yes, that is a new offset growing. Not much you can really do to help it but cut off the top where the damage is. Keep it shaded until it drys and then give it much more ...

I'm not too sure how much variability these plants have, but it looks like possibly something else mixed in. The flower petals are usually long and thin and bright yellow.

I agree with PeterB, not leptacaulis. C. kleiniae is a very likely suspect, although Hidalgo is too far south for kleiniae naturally, according to distribution maps. Here on CactiGuide, no Cylindropuntia at all show up for Hidalgo. If it means anything, my C. kleiniae is currently in bloom and has b...

No Sulcorebutia krainziana, but there is a Rebutia krainziana. http://www.cactus-art.biz/schede/REBUTIA/Rebutia_krainziana/Rebutia_krainziana/rebutia_krainziana.htm" onclick="window.open(this.href);return false; Sulcorebutia steinbachii? http://www.cactus-art.biz/schede/SULCOREBUTIA/Sulcor...

A big ole clump of Echinocereus reichenbachii!

Rebutia krainziana as i know it is a spineless plant, or they are extremely short it appears spineless with bright red/orange flowers. I don't see any thing in common with your plant. I'm certainly no Rebutia/Sulco expert, but it's more affinis to canigueralii than krainziana to me.

Might be Ecer. russanthus var. vulpis-cauda?