CPU progression seems really bad. As for progressing user controlled teams I'm not seeing the issues you describe. For 1. and 2. you are probably focusing on OVR ratings too much. WR is a more dynamic so different player types will rate differently (OVR) in different schemes. Upgrading a WR across the board can be difficult since there are so many attributes but individually each attribute is fairly cheap. So I wouldn't say it's more difficult to progress WRs rather that you have to put more thought into it.
D-Line on the other hand only have a few meaningful attributes, but they tend to cost more. So as long as they accumulate XP it's easy to progress them.
For #3 I can't really say much either way. I haven't really progressed any LBs too much. All LBs seem difficult to progress, primarily for me because I play with 6 min quarters and they don't get enough tackles and that seems to be the primary XP source for LBs.
4. and 5. I don't see an issue.* Average CB tackle is 55 which seems just about right to me. Only about 20% of CBs have tackle under 50 and about 1/3 are above 60. To me average CB tackle between 50-60 with good being 65+ seems right. CBs are supposed to tackled WRs, most of whom don't have great elusiveness or trucking to break tackles.
Safety man coverage seems about right to with a average of 61. That includes both FS and SS. This makes almost perfect sense if you compare it to route running as 61 lies almost exactly in between the average HB route running and WR route running. A safety should be able to adequately man cover a HB and should have some trouble covering a WR, on average of course.
*ratings are from the original ratings reveal spreadsheet.