You've stated your point, but I find it at odds with current reality and therefore disagree. In general and in theory what you say makes sense but it just doesn't jive with the obvious state of things...
Again, the Big 4 are the Big 4 on all surfaces for the most part; especially since 2008 (perhaps not before especially with Nadal vs Federer - Nadal was not making it deep into the big HC tourneys as much pre 2008, which helps explain why the two have met on clay so often)...
Therefore it is logical to assume that a change in surface distribution would nearly equally affect H2H surface meeting distribution. As you've already stated Nadal, Murray, and Nole have a pretty even distribution amongst their H2H meetings regarding surface (most likely because they are closer in age and matured together). The only relationship amongst the Big 4 that would appreciably change with even surface distribution on tour from the beginning would be Federer and Nadal (mainly because Nadal was a clay court phenom from the beginning and was meeting and beating Federer on that surface at the start but took longer to become competitive on HCs)...
So basically with more clay court and grass tournaments; Nadal would have more of an advantage. Not just because he's better on clay and grass than on HCs but also because Nadal's schedule would not be so compressed between the French and Wimbledon (less injuries)!
Well I'm just saying more HC doesn't automatically lead to meeting rivals more often on HC than clay. Looking at Djokovic and Nadal they have only met twice more on HC than clay DESPITE HC far outnumbering clay. The H2H surface distribution does not reflect the tour surface distribution with either Federer or Djokovic, it probably does with Murray.
Ironically, when Nadal did start making HC finals all the time 2011/2012 was when fans like you said he was playing worse than usual, even on HC.
Also I don't really think there is enough evidence for what would happen if Nadal played on grass more. How many times would he lose to random people like Tsonga, Lopez, Kohlschrieber, Haase, Petzschner, Soderling (all people who have beaten him at queens or been 2 sets to 1 or 0 up against him at Wimbledon) Nadal has sometimes struggled with lesser players on grass, but usually comes through at Wimbledon because he has a champion's mentality. Easier for lesser players to pull of an upset in best of 3.
Also there is enough clay, no need for more. Just put half the HC tournies on grass and it would be pretty even in a 3 way split. I find that people wanting more clay as well as grass at least subconsciously want to favour Nadal and have more clay than anything else, and don't really want a 3 way even split.
Likewise, anyone facing reality can see Nadal has done an awesome job of leading the H2H agaisnt all his rivals, so there is no need to make out that he is also unlucky regarding H2H. As I said despite less clay, he's had PLENTY of clay meetings with Fed and Djokovic (not so much with murray because Murray is not so good on clay) What do you want? To have twice as many meetings on clay than any other surface? Despite HC outnumber clay, Nadal's meetings with Fed and Djokovic have NOT reflected this at all. He's met them on the surface he owns a fair amount of times. You forget it's not like he's meeting them in every masters event. Look at it since 2008 which many people say was the start of his prime.
2008
Met Djokovic 2 times on clay and 3 times on HC
Met Federer 3 times on clay and 0 times on HC
2009
Met Djokovic 4 times on Clay and 3 times on HC
Met Federer 1 times on each
2010
Met Djokovic 2 times on HC and 0 on clay
Met Federer 1 time on each surface
2011
he met Djokovic 2 times on Clay and 3 times on HC
Met Federer twice each
2012
Met Djokovic 3 times on clay 1 time on HC
Met Federer 2 times on HC and 0 on clay
There's not much bias towards HC at all.
Totals
vs Djokovic 12 on Hc and 11 on clay. Pretty even.
vs Federer 6 on HC and 7 on clay. Again pretty even. The total adding both players is 18-18.
Hence the surface distribution as biased towards HC as it is, has resulted in a H2H played fairly evenly over clay and HC the past 5 years. You can see this as a fact. Their meetings since 2008 have been evenly spread over HC and clay. More clay would simply mean that Nadal played them more often on clay than hard court IF they proved better at reaching Clay semis/finals than he did at doing the same on HC. Which is an odd thing to think fair if you also complain that the bigger amount of HC gives an advantage in winning titles. A case of wanting an advantage in the H2H (in terms of playing on Nadal's favourite surface more than the other player's favourite surfaces) but then bemoaning the advantage of HC players with regards to titles.
Lastly, one of the big things you forget is, with a dense period of 3 masters and a slam on clay, it's likely that the top 4 contest the latter stages. With so much HC spread over the whole season, they don't all make the latter stages of (or even enter) the same events at the same time. Thus the HC meetings remain the same as the clay ones.