Hello there .. getting a question answered about a specific light(-s) ist not allways easy but you seem to be in luck ...
..... "also a flashy seems more focused on seeking knowledge instead of sharing it" (in my experience)
I don't know or have the AB T28 but I do have the T27 and even based on this (lack of) knowledge .... I would say that for a (more) general purpose light you are probably better of including the T27 within your selection, it's about 30 % lighter than T28 and also considerably smaller ..... Most important however, in my opinion, is that the T27 comes in NW and that looks alot better when you want to enjoy the colourful surroundings nature has to offer.
That said comparing the T27 and the HT18, which I both have, I would also say that the HT18 has more and better general purpose advantages that you also seem to be looking for ? (<= "enough to see the ground in front of me")
As to your questions about the HT18, the internal diameter of the HT18 is 22,05 mm and for the T27 it's between 22,05 and 22,1 mm; so the difference is only just measurable/noticeable with my Mitutoyo .....
As to how the HT18 knows a 18650 or 21700 is "loaded" .... in the package there is an adapter included for 18650's and this basically tells the light that an 18650 is present.... without the adapter the 18650 would not be able to make "contact" to say it blunt .... because the 18650 is shorter an thinner than the 21700 !
Just for the record ... the package of the AB T27 comes with a similar adapter for 18650's !
The difference between the HT18 and T27 is virtually nothing both in weight (< 5 grams) and length (about 1 cm for internal charging)
I think the HT18 has other advantages .... comes with green and red filter, with green throw is still reasonably good but wuth red it's about 30 % of white.
The Fenix is called HT which basically stands for Hunting Tactical light and that's also what the colors are for .... most animals are more or less color blind when it comes to certain colors ..... allthough we also are animals ... most "wild" animals are better equiped for seeing shades of grey or light and dark compared to us (human) animals ...... 
....
Here is a good review you should read that also goes into the use of the filters ! (use the translator in your webbrowser)
In the review you will read why the HT18 actually doesn't really have a TURBO mode (!) the sequence is basically ECO-LOW-MED-HIGH and that's because high runs alot longer than the usual TURBO mode as it's not as the name implies a "short term turbo boost for a (few) minute(s) or so" .... HIGH actually runs, based on the used battery used, for upto 30 minutes or so with up to 90% of HIGH (= 90 % of 1500 lumen).
This 10 % of initial drop is not noticeable for your eyes ..... and probably meant to provide some baseline for the regulation which is almost perfect.
A similar allgorithm is used by Eag(le0Tac which starts at 100 % and after 2 to 3 minutes it reduces the output to around 80% of turbo which has proven to be more/easier sustainable, wthout noticeable heat buildup, and thus the regulation will have an easier task => without having to reduce the power quite suddenly after TURBO has caused a rapid heat buildup.
The advantage of this procedure ist that the output doesn't have to be regulated to far down below the point of equilibrium ..... so the excessive heat from TURBO is be able to dissipate .... after which the power can be turned up (a bit) again ..... to or even above the point of equilibrium ..... so in other words it's easier for the regulation after startup to drop to a point, of about 80% (= equilibrium), that has proven to be quasi-stable during testing ..... this will thus prevent the occurrence of the well known "JIGSAW" curve that you often see in runtime graphs.
You should also realize that the smoother/easier the regulation is or better said "the more efficient it is" => the more energy from the batteries goes to powering the light instead of regulating it ! (just simple logistics really)
In reference to the T27 which comes in a nice NW ..... the HT18 also comes in NW ..... but comparing the both I would say that the T27 is more WarmWhite and the HT18 mor NeutralWhite ......
So in other words the HT18 is a bit to "cold" and the T27 is maybe a bit to "warm" ..... but that's mainly when you compare them in real live side by side ..... seperately viewed they seem almost the same ?
If you have more questions with respect to the T27 or HT18 I'll elaborate appropriately if I am around .....sort of retired .... tried to inform people but backfired a bit you could say ....
I hope this might have helped you with some of your queries .... as to go "right or left"...... in reference to the pictures supplied with this post ?
Update --------------------------------------------------------
With respect to the adapter .... it seems the adapter has quite a big "nipple" so to say (1), it actually looks like a completely "metal" (human) nipple with both a raised small surface and an even larger low contact surface , both are about three times the size of the normal PLUS pole/contact of a regular button top battery you might use in the HT18 (2)
Inside the head of the HT18 there is a ring of contact's (4) around the mainspring inside the head of the HT18 (3).
My guess is that (2), the plus pole of the battery, supplies power to the big nipple of the adapter (1) which in turn, when used, makes contact with both the ring of contact's (4) aswell as the main spring in the head of the HT18 (3).
So if the both contact's (3+4) in the head of the HT18 make contact with the plus pole of the battery, which is the case when the adapter is used the HT18 will know an 18650 has been "loaded" .....
If only the mainspring in the head (3) of the HT18 makes contact with the plus pole of the battery the HT18 will know a 21700 has been loaded (in this case the ring of contact's will touch the black insulation around the plus pole of the 21700).
So it's all in the contact area that's established in the head of the HT18, on the plus pole side of the battery or in the head of the HT18, the bigger it is, with adapter basically, an 18650 will be loaded and when it's smaller or "normal", meaning just the spring makes contact, a 21700 will have been loaded.
I hope my somewhat elaborate explanation makes sense .... a little drawing might have been easier ..... as usual a picture tells a thousand words.......?
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Cheers,
Martin
PS> I am having some issues with the software of this forum .... "things" don't seem to work or suddenly disappear ?
