I think a lot of people worry about this MUA thing for nothing. Part of the problem with not being able to bend in addition to swelling is that your muscles hurt, your ligaments and tendons hurt. You have to work through that discomfort to get a good quad and hamstring stretch. It may "hurt" but it isn't going to hurt you. Even when moved gently it will hurt but you will know when to stop and when to take some nice deep breaths and go a tad farther. Suck it up. You got through surgery you can get through this. Some days will be better than others.
There are plenty of exercises that you can do to achieve little gains without going over the top. For flexion, try sitting in a chair and doing your leg slide when you get to the point where you don't think you can go any farther, lift your heal off the floor, you will get a stretch in a slightly different muscle and you may be able to slide another inch. Work on it and give it a try. Do this several times a day.
For extension, lay on your bed or sit in a chair and put a rolled up towel under your ankle and stay there for 5 minutes. Do this a few times a day. Another angle is laying across the bed on your belly and letting your leg hang. It won't hang far but you will feel it stretching. Stay this way for a minute or so and work to 5 mins. It will be difficult to get out of this position because it will be uncomfortable but do it anyway and bend it slowly when getting up. Do this a couple times a day. Patience, perseverance and time along with those handy dandy pain meds.
You, me, and all of us suffered an inordinate amount of pain prior to this surgery and can handle a little bit of pain to work through an exercise or two that will benefit you beyond belief in the long run.
This is my 2nd TKR, I'm 11 weeks out and my range of motion is 125 - 0. I probably won't get beyond 125 but frankly my dear I don't give a .... darn. I'm a fast healer but that doesn't mean it didn't hurt like heck doing some of these things and that I don't still have pain, healing takes months but you put yourself through the surgery now do what you need to in order to gain your life back.
Harsh, I know but life is harsh and we are our own worst enemies and biggest advocates. Only we can get this done. Stop worrying about something being wrong and focus on what's right. If there is something really wrong I believe you will know it.
When all else fails - have a cocktail!
P.S. This message isn't intended for anyone in particular and no disrespect intended.