Sorry it took a while -- I wanted to pull out my maps before I answered. Based on what I read on that thread, the answer to your question about my training hike covering the questionable sections is "yes".
I can also tell you this now: The trail from 4th of July Lake to Summit City and then 2 1/4 miles past the junction with the Horse Thief Spring trail is still a part of the trail system but infrequently maintained. So how bad it is really depends on when it was last maintained. Any trail which may have connected from approximately 2 1/4 miles beyond the junction with the Horse Thief Spring trail to Camp Irene is not a National Forest System Trail (if it ever was, I don't know) . This means the route is not maintained, ever. Expect slower travel.
I knew this was true, but needed to look at the map to pinpoint the segments.
The good news is that from Camp Irene to Lake Alpine the trail is a maintained system trail. I haven't done this section, but I do understand that its steep and causes problems for stock, which won't affect your travel. You might consider checking with the ranger stations to find out the condition of this portion of trail. I think its maintained by the Stanislaus NF.
If you want to find out about the portion from 4th of July Lake to Summit City, you'll want to call the Amador Ranger District on the Eldorado NF. The trails from Meeks Bay to that point should all be in fairly good condition as they're either in high use areas or part of the PCT/Tahoe Rim Trail. I've been on most of them in the past, but not recently.
A word on infrequently vs. frequently maintained. Even the frequently maintained trails can go a while (seasons) without real maintenance or brushing these days because the workforce & budget just aren't there. They do what they can, but have to prioritize where they focus based use levels and on where the known problems are. The frequently maintained trails are assessed on a more frequent basis (with a regular schedule: certain percent of the system each year), so those trails tend to be in better shape. The assessment schedule for infrequently maintained is much longer, . . . you get it.