This is definitely true - the blinds outside the proper entrance to the park (but still within the entire boundaries) would offer the chance to see some more difficult species up close, something that isn't really offered around Khao Yai NP. Blue Pitta is also actually fairly easily seen at Khao Yai NP, although December might not be the best time of year - they are resident of course, though.
Crowding on weekends within Kaeng Krachan can also be a problem, especially around the Ban Krang campgrounds, and with the road up to Panoen Thung still closed, with no re-opening dates announced (it's been closed for a few years now), it means more people in a smaller area. Additionally, with Panoen Thung still closed, the opportunity for 'look-out' type scenery is greatly reduced at Kaeng Krachan, whereas at Khao Yai there are several sites with look-outs over valleys etc., as well as many more sites to actually visit within the park.
This is true; however, at both parks you are prohibited from walking/driving around aside from within the campgrounds. Khao Yai does offer night safaris for a small fee, however, and herping etc., is supposedly still not too bad around Ban Krang campgrounds at Kaeng Krachan.
And yes, once inside the 'proper gates', camping is the option.
This is true of most NPs in Thailand - often guides are required even for the most tame trails; however, Khao Yai NP does have the best trail network of any park in Thailand (that's not saying a lot unfortunately), and several can be walked alone. The trails that do require a guide can be attempted alone, but you will get in trouble if a ranger finds you on one, and at Khao Yai they do patrol these trails.
During the week is absolutely fine. Given the size of Khao Yai NP, and the many separate sites within the park, you can actually find quiet places on weekends as well, but the campgrounds, visitor centre/restaurant area and major trails are usually busy, but even then, they are busier later in the morning - you can still get up before the crowds and see a lot.
In my opinion, if you're after more than just birds, than Khao Yai NP is probably better, and with your time-frame being December, then many of the colourful forest birds (broadbills, secretive forest kingfishers, pittas) at Kaeng Krachan (and Khao Yai) will be harder to find (but not impossible) because they are more active from April-June during their breeding season. And given the experience you've said you're after, I'd go for Khao Yai, especially if Panoen Thung at Kaeng Krachan is still closed and sitting in hides/blinds in not your thing.