Personally, I prefer just losing guess to having a more subjective judgement of whether someone is using dubious methods of guessing range. I am sure that you have called people up for surfing at some stage and had the surfer loudly deny any wrong doing. I can see the same happening with guess ranging, without a great amount of benefit, if rules preventing assisted range estimation were introduced.
If your idea of using the lowest of 2d6 for AP with an ordnance weapon were used (when the centre misses the target), then regular blast weapons should be less effective. That is why I was suggesting 3d6, take lowest.
As for the entire hole needing to be over the target, I really don't know. It is such a small location that I can see people having problems with judging the exact direction of scatter and suchlike. My instinct would be that the centre of the template is the centre of the hole, so that it will hit the vehicle if the centre of the hole is over it.
If you can't afford a full battery of things like D-Cannons then they will be less effective, you just need to decide if that is worth it or not.
My own experience against Biovores and Spore Mines is that if they miss the target and don't get set off by Synapse that I usually have the choice of taking the explosion (if I don't think it will do too much), or shooting it (often with a unit that has moved to prevent the mine exploding next to it). I don't find that floating Spore Mines really do much more than they would if they always exploded, perhaps even less.
I don't think that the changes will mean that artillery tanks will be placed in the open regularly, but some people might try that option. Basically, I think it will tone down the effectiveness of artillery, which as a Dark Eldar player, I have very limited experience with (as we can't get any), although it can be very damaging to my force.
Against a capable guesser, dropping the template on top of a fast moving vehicle or in the same place as last time, with the current rules isn't terribly different. For a poor guesser, it will make a difference, but I tend to assume perfect guessing by my opponents and get a pleasant surprise if it isn't right on target.
I agree that my suggestion about single d6 scatter for shooting at the same target as the previous turn could get a bit too messy. I was thinking the same point on the table, shooting at a previously immobilised vehicle for example. Basically, I wanted a way of including some crew correction.
My comment about Spellbound's record wasn't intended as a cut on him, or anyone else. It was simply that if his army wins all or almost all of its games, unless there is another factor (like skill disparities) then it should be toned down. My observation would be that Rhino Rush style armies require less skill to use effectively than a shooting based army. As such, I don't think it is unreasonable to reduce their effectiveness slightly. The big question is really whether things have gone too far or not. A few dozen more games with them and I can provide a better judgement on that one.
As a side note, I am in the early stages of starting up a largely Rhino based Slaaneshi army. With the TVR firmly in mind, I am confident that I can win with it, although perhaps less easily than with the current rules in place.