JDSTER, your package arrived in the nick of time, thank you 
The poor kangaroos, we have quite the menagerie here already, I'm sure they'll fit right in with the snails, sheep, and lion (who have I forgotten 
?) Sallie, I used to suggest the UB CO, with reservation, to those who wanted something easily picked up in Boots but these days I tend to steer those requests to the Simple Cleansing Oil as it has a good INCI (just never used it myself).
Sophie, I have a Retinoids info sheet in the works, I just halted on it as there was suddenly a surge of interest in it because of that terribly misinforming BBC programme and I feared having it in the OP would add further to the misconceptions as it was highly likely that would be all that was read.
Anyhow, there are a few more forms than you listed, they vary mostly with how sensitising they are and how many steps are required to transform it into retinoic acid.
The percentages are even more confusing, partially it's because if they gave it all way they are essentially giving away their formula. The other aspect is that most cosmetic companies are sold to (and equally subject to questionable marketing) and often buy something in a solution, so the exact % is almost impossible to tell. Just read this thread on the SA Reddit and heave a big confused sigh.
Hydroxypinacolone Retinoate (HPR) is the one used in TO and a great many other products. It appears to be heavily marketed towards cosmetic companies, all research comes from the manufacturer so that is a touch suspect (though R-R has the same issue) in a sense and even in its own marketing material it suggests formulating it with other anti-acne ingredients for it to actually have effect for acne. The first hyped product with HPR was probably Sunday Riley Luna, which was a complete dud for many, required very specific use and only had a small group of people actually see good results. Its main selling point seems to be resurfacing mostly (very little, if any, anti-ageing is mentioned in the manufacturer's info), which makes it not too dissimilar to an acid toner.
Squalane is indeed highly occlusive (not far from Mineral Oil on the TEWL scale) so in that sense, it could be less helpful since you also have issues with MO, most likely on account of its occlusive character rather than the ingredient itself.
I was rereading this post on Retinyl Retinoate, bookmarked from back when I was researching the Verso and I noticed that this ingredient specifically boosts HAS2 (the enzyme that synthesises hyaluronic acid) which could explain my quick results with the Medik8 as my dehydration is more a genetic predisposition than the result of product abuse/ageing/etc. and would also explain the feeling that I felt that TO was making my dehydration worse.
This guide is also a good overview.
Sallie, are you using the extra strength BHA from PC? That one would definitely be a mismatch with Differin as that would dry you out further. Often BHA is recommended to use (usually as a cleanser in the AM) with prescription retinoids to help soothe the inflammation, so they're not mutually exclusive by default but if there's no specific need for it then I wouldn't bother using it.
Sophie, yes on alternate nights I use the Verso Eye Serum and Medik8 together (on separate areas, obvs) although the Medik8 marketing is a bit odd as they highlight improved crow's feet area in before and after shots which suggests you could use it in the eye area and it doesn't explicitly say not to not use it on the eye area either, so I assume they just haven't made the investment to test it (or are doing so and will then probably sell it as a seperate product at some point).
In terms of what retinoid to pick, it depends where you are at. I wouldn't recommend someone with advanced signs of ageing and little to no sensitivities to bother with OTC retinoids if they wish to see results anytime soon. 30s-40s preventative use will have different needs to someone who also has acne, rosacea, etc. General guide is try the strongest you think you'll be able to tolerate, impending summer is something to take into account, a synthetic retinoid makes more sense as they make you less sun sensitive. So perhaps try an encapsulated retinol, HPR and similar, I don't know if I'd jump in with R-R (though I did, and I do regret ever bothering with anything else tbh) unless your skin sensitivity would make that the most likely only candidate.
mint, I'm pretty useless with hair products, I tend to cling on to whatever works and then despair when it stops working for me (currently in such a crisis). I've attempted to understand the boggling science behind it all but it's a lot tougher to tackle than skincare, I use SLS-free by default as that just works better for me, but I don't mind some sulphates in it, just not SLS and I'm not a huge fan of SLES either. Using conditioner on the roots strikes me as odd though, that would leave me with very dirty looking hair very quickly and I'd only be able to use 'light' conditioners as a result that then don't really tackle the continual dryness. Can you tolerate fragrance in shampoos and conditioners? I sampled the Kiehl's Damage Repairing & Hydrating Shampoo & Conditioner which wasn't terrible for that type of shampoo (I usually end up looking like a poodle with those), it's not the 'perfect' combo for me to use long term but it did do a good job of holding on to moisture and water and not turning it into a dry frizz ball.