The initial grant (the 4hours) for estimating and planning the work to be done, helped a lot to structure the remaining work and identify issues. Once the issues were identified, many contributors chimed in and helped solving the issues for converting the remaining form validation converted to the entity validation API - together with frequent reviews this helped already a lot speed up progress. The issues covered were:

#2247085 Constraints cannot be added to configurable fields:
After identifying, repeated reviews and the great work of larowan and berdir were enough to help driving this one home.

#2406103 Remove hook_node_validate() and hook_node_submit() because they bypass the entity API:
Again quick success, after identifying work, repeated reviews and the great work of berdir.

#2403817 Feed entity validation misses form validation logic:
After identifying, larowan basically took care of it with little help!

#2405943 User entity validation misses form validation logic
While taking a closer look at this one, we found quite some remaining unconverted logic - plus a special rather hard to solve issue. We decided to create a separate issue for a hard to solve part: #2418119 REST user updates bypass tightened user account change validation (see below). The main conversion was solved in this issue: Thanks to the help of klausi's and other contributor's reviews, there was good progress and we were able to solve the issue.

#2403823: Menu link content entity validation misses form validation logic:
Got solved by re-factoring that happend meanwhile elsewhere!

#2403485 Complete conversion of comment form validation to entity validation:
The existing form validation was hard to untangle. Larowan helped with an initial version and was able to review, thus we were able to solve it.

#2418119 REST user updates bypass tightened user account change validation
Together with berdir, klausi and me discussed possible solutions to the problem. With berdir working a lot on the code, we've been able to make good progress here - there is still some work left though.
Besides those issues, there are the tickets for solving remaining issues more at the architecturial level. #2395831: Entity forms skip validation of fields that are not in the EntityFormDisplay is the main issue for that, which was divided into further sub-issues as necessary. Thus, all releveant issues here are:

#2429037 [PP1] Allow adding entity level constraints
We made good progress on this and have have a working, RTBC patch. However, to avoid further future API changes we postponed the issue on the following one:

#2346373 Data reference validation constraints are applied wrong
Previous, this was only a major bug, but it turned out we need to solve it first for avoiding API changes in the above issue. It has a working patch, but progress got stalled on discussions on the best way to do it. (Discussion in progress)

#2105797 Add CompositeConstraintBase for conditional validation involving multiple fields (previous: Figure out the best way to do conditional validation)
The issue is outlined and work planned, but work has not started yet.

#2395831: Entity forms skip validation of fields that are not in the EntityFormDisplay
The general issue was discussed with the maintainer of the Symfony validator component (Bernhard Schussek), with whom I was able to find a good solution to the general problem. There is already a first patch implementing the solution, but there is some more work to be done. Plus, it needs the other issues for completion. So in general, the plan to solve the issue is pretty clear now and half-way implemented - it still needs some more work to get it done though.
Summarizing, I think the the grant enabled me to take the time necessary to focus on a rather complex topic like this, what has been very helpful for identifying the work to be done even. After that, the grant enabled me and klausi to provide contributors the necessary feedback in time and helped us to get the larger conversions (user, comment) done rather quickly. For the architectural issue the grant was critical for being able to take the time needed to focus on it, think it through, outline, plan and weighing up the pros and cons of possible solutions. Imo it helps tremendously to speed up progress here, as else I guess this would have taken a hard-problems discussion at a drupalcon or so to move on with it.

Regarding estimations, I think previous estimations basically worked out thanks to the great help of contributors with the reamining conversions. However, I under-estimated "#2405943 User entity validation misses form validation logic" as I did not correctly identify all work needed here at the beginning, so some time saved in the other issues got spent there.

Finally, I've also estimated the remaining work in case you'd like to continue with the grant:

Tasks Estimate low Estimate high:

#2429037 [PP1] Allow adding entity level constraints 0h 1h
#2346373 Data reference validation constraints are applied wrong 1h 4h
#2105797 Add CompositeConstraintBase for conditional validation involving multiple fields 3h 8h
#2395831 Entity forms skip validation of fields that are not in the EntityFormDisplay 4h 12h
#2418119 REST user updates bypass tightened user account change validation 1h 8h

Total: Low 9h, High: 33h

Besides those issues, I'd be happy to take care of another entity validation critical:
#2343035 Upgrade validator integration for Symfony versions 2.5+ Min: 4, High: 12h

Best regards,
Wolfgang