First off, I completely agree with this, I think one of the biggest issues with League right now is that marksmen have been conspicuously spared from the waves of drastic reworks we've received in the name of counterplay, when the class has always been among the least interactive in the entire game. The greatest change we've got was during the Marksman Update, and even then it only focused on giving marksmen more variety and options (not to mention straight-up buffs), instead of improving how the class could be outplayed by others. Reliability is not an excuse, and I don't think it's even really a valid justification: no class should be defined by their reliability, as reliability is one of those factors that will always be preferred over all others (especially because it implies lack of outplay potential against the player). Marksmen may rely on one of the most reliable mechanics in the game, i.e. autoattacks, but that should be all the more reason to make the rest of their power all the more unreliable. Instead, marksmen tend to have some of the most reliable abilities in the game, including on-demand steroids, cheap and easy free-targeted mobility, and point-and-click crowd control (which itself includes hard CC). This urgently needs to change.
One thing I appreciate in the OP is that it lists the different ways different classes have counterplay, and I very much think that updating the gameplay of marksman will require giving them their own unique counterplay: delays could help on some effects, but can't hold up all the power of a class based on persistent, point-and-click ranged damage. The same goes for skillshots, and kiting is obviously not an option. What marksmen need, I think, are more conditions to their power, where they can't access the kill potential they have now unless they work for it. Part of this could be through the implementation of win conditions, i.e. conditions where, if met, you get access to a power boost (e.g. Ashe's Q, which requires autoattacking a bit first). The flipside to this would be loss conditions, where if the enemy fulfills a certain requirement, the marksman loses power (Miss Fortune's W can be outplayed by forcing her to focus a single target, for example). Many abilities are likely to be a mix of both, such as Draven's Q, which has both a win condition (i.e. go to the right place to catch your axes while also fighting your opponents) and a loss condition (impair Draven's movement at the right time and he won't be able to catch his axes, which will cause him to lose a ton of power). Additionally, double-edged effects could be a great way of giving marksmen immense power with comparably huge tradeoffs, which the enemy could exploit: Jinx's Q is a miniature version of this, where she trades off range or attack speed in order to access the other. The idea behind Kog'Maw's reworked W, in which he'd gain massive DPS in exchange for total immobility, had a great idea behind it, if it weren't for Kog's damage output killing even tanks before they could approach him.
Point being, the foundations upon which marksmen can be made more interactive are already there, they just need to be developed further and ramped up to 11, in that much more power should be stowed away behind those effects: marksmen right now suffer from a tremendous lack of variety in gameplay, and are back to competing with each other on raw stats (in this instance, with who gets to abuse Ardent Censer the hardest), in large part because ultimately all of their power boils down to just raw autoattack damage via the same items and stats. Unlike abilities, which are all completely unique and heavily customizable, autoattacks tend to be extremely consistent, and usually vary only in specific numbers (e.g. AD, AS and range), so while marksmen should be dependent on them to output power, they should be using them to do interesting things, and not just attach as much pure, generic power onto them as they can. This will mean taking a look at many of the marksman steroids and abilities we currently have, and implementing much more drastic changes to the way each of them autoattacks, so that they really get to contribute in a completely unique way (Sivir's Q could easily be a permanent modifier to her regular basic attacks, for example).