Having difficulty making the data in the below charts to apply to daily life. What is the right interpretation and implementation? How are the “in vitro” results of the below charts relevant to levels that cells would experience "in vivo" after the consumption of caffeinated beverages?
In the mentioned study, they tried to support the evidence from earlier studies in which caffeine consumption was associated with skin aging and slow wound healing:
Our research confirms our earlier observations that caffeine may have an adverse effect on the wound healing process, as well as on the aging process, of the human skin.
...and by "earlier observations" they mean another in vitro study.
In the study, they used 1, 2 and 5 mM caffeine solutions. 1 cup (~250 mL) of brewed tea can contain 95-165 mg caffeine (source), which makes it 2-3.4 mM, so much like the solutions used in the study. When you drink 1 cup of coffee (let's say, 2 mM caffeine solution), the caffeine, which is water- and lipid-soluble (StatPearls), will be dissolved in the body water and fat, which can together make ~50 kg in a 70 kg person. So, you would need to drink ~200 cups of coffee (~20 g caffeine) in ~2 hours (caffeine half-life is ~5 hours - source) to achieve 2 mM caffeine solution in the skin cells.
I haven't found any actual human clinical trial in which caffeine consumption would be associated with adverse effects on the skin. In a recent review: Coffee consumption and health: umbrella review of meta-analyses of multiple health outcomes (BMJ, 2017), they don't even mention skin or wound healing and regular drinking of 3-4 cups of coffee per day was not associated with an increased risk of various health conditions (except in pregnancy).
How can one relate in vitro studies of caffeine (dose expressed as concentration) to dietary intake of caffeine (dose in mass)?
Caffeine rapidly diffuses throughout the body's water, both intra and extracellular.
Therefore, since the caffeine will be relatively evenly distributed, you can calculate the amount of caffeine needed to reach a given molar concentration within any particular cell by knowing the total water in a person and the molar mass of caffeine (194.19 g/mol ). A 72 kg person has about 40 liters of water in their body.
Combining those numbers, reaching 1mM requires 7.7676 grams of caffeine. Reaching 5mM requires 38.838 grams.
How are the “in vitro” results of the below charts relevant to levels that cells would experience "in vivo" after the consumption of caffeinated beverages?
Wikipedia puts the LD50 of caffeine at 150-200 mg/kg, or 10.8-14.4g for our hypothetical 72kg man. These numbers are likely very approximate as there isn't going to be a lot of well controlled clinical data on caffeine toxicity in humans, but someone reaching those concentrations stands a good chance of dying from caffeine poisoning.
For that reason I don't think these results are relevant to normal caffeine consumption. I interpreted the high concentrations used in that study as a deliberate attempt to induce cell injury, not as a means to simulate normal caffeine consumption.