I agree with you overall. But I would say that for Nadal, there is a big gap between his best surface (clay) and his next best surface (grass?) but a very small gap between his second-best surface (grass?) and his weakest surface (hard?) Indeed, I am not 100% sure that grass is stronger for him than is hard. For Djokovic, there is only a small gap between his best surface (hard) and his next-best surface (grass) but there is also a small gap between his next-best surface (hard) and his weakest surface (clay). I think it's because of that fact that some Nadal fans try to deny that Djokovic is more versatile by saying that Nadal has no surface as weak as Djokovic's clay. Hence we often see the factoid about Nadal winning two Slams on every surface and Djokovic not doing so. There are three big problems with that argument, however:
1. There are two Slams on hard courts and only one Slam on clay. So, Nadal had more opportunities to win two Slams on hard courts than Djokovic did on clay.
2. Relatedly, the Australian Open and the US Open are different Slams and should not be subsumed into each other. Nadal only has one title at the Australian Open. Until he gets a second, he should not be counted as doing better there than Djokovic has done at Roland Garros.
3. It involves double counting Nadal's clay-court dominance. You are absolutely right that Nadal is a lot better in his speciality and he should get credit for that. But the credit for that is pointing out that Nadal has 12 Roland Garros titles to Djokovic's 7 Australian Open titles. The reason that Djokovic only has one Roland Garros title is that Nadal is so dominant at Roland Garros. If it weren't for Nadal, Djokovic would almost certainly have at least two Roland Garros titles and likely more. If Nadal had to play someone as good as he is on clay on either grass or hard, grass/hard Nadal would be very unlikely to pick up multiple titles at the event. Nadal should not get both dominance points and versatility points out of his Roland Garros record.
Don't even get me started on the slams. It's not just the fans that do it, it' s the journalists too. You know, the "oh Nadal just needs two slams to match Fed". I couldn't care less about Fed personally but I cannot stand hypocrisy, dishonesty or lazy thinking. Total # of slams is not the only reason why Fed has the best record in slams. There are 4 slams. Fed has a dominant record in 3 out of the 4 on top of winning all 4 (he has 5+ in 3 of the 4: tie record at USO ( 5), absolute record at W (8) and close to record at AO (6). That is a dominant record in slams overall.
Nadal has 12 RG and 1 to 3 titles in the other 3 slams. That is domination at 1 slam. 1 slam out of the 4 cannot equate overall domination, no matter how much one dominates at that one slam. So, no, Nadal doesn't have to just win more slams to be in the same conversation as Fed, what he needs is to win more titles at the other slams.
ETA: about your other points, I'm not sure I understand your surface argument. Djoko's winning % on clay is higher than Nadal's % on either hard or grass. So how does that translate into Djoko's weak surface being weaker? For the titles in slams per surface, yes, I've argued it in a lot of other threads. You just have to take into account that 50% of slams are on hard and 25% on clay. Not fair but there 's nothing one can do about it, especially not claim that I'm just gonna pretend 25 =50. And overall, it's the same thing, there are a lot more (tier 1) events on hard than on clay or grass, so unless you factor that in, you are just trolling.
, which one of the 2 points to extreme consistency across the board and which one points to extreme peak in specific events ? I know it's hard but you can do it 
