Take a peak at the straps as well and make sure that they are nice and straight with no twists in it, as that can be making it harder to tighten. My sister recently was having that issue with her one car seat and it was because her husband twisted the strap and had it connected backwards so that twist was making it impossible to tighten the harness properly.
This is my little guy in the Scenera when he was 4 months old (and about the same weight as your daughter):
http://i1140.photobucket.com/albums/n575/jacqui276/kalebscenera.jpg
The straps go tight enough that it should fit and I always found it to be a smooth pull to tighten them. If you can get the Scenera to function for you, it may get your daughter to 2 if she is small but it is often grown out of quickly both RFing and FFing. Height is a bigger issue than weight limits with this seat and the one that I had had a maximum of 32" for height for RFing, so my 11 month old would be extremely close to outgrowing it already (he is big though).
The only issue that I have heard with the Graco MyRide is that some people have been having problems with the Graco buckles. Some of them have been malfunctioning and making it very hard to undo them, which deters some from buying Graco seats. Others have used them with no issues but for those that have had issues, it is obviously frustrating. I had one briefly (returned it as it didn't work in my 3 across situation) and found it easy to install and the harness adjuster was super smooth. I didn't have any immediate issues with the buckles, but also didn't own it long enough to see if anything developed.
I have never owned a Britax seat but the general consensus seems to be that they are easy to install and use. Their only downfall is that they have a small shell so are outgrown sooner than some of the taller seats on the market and have less leg room than some others. If your daughter is small though, that should be much less of an issue for you.
The seats that I have heard of for having an issue with pushing the child's head forward is the Graco Nautilus, and Cosco Guide 65. Not everyone finds it to be a problem though.
The Harmony V7 is one of the most inexpensive ffing/booster seats that is available here (it is at Walmart). I found it very smooth to adjust but the assembly of the seat was a little bit annoying (it came in pieces in the box unlike any other car seat I have ever bought). I found it ok to install with LATCH and a little bit annoying to install with seat belt but with some help from here got the hang of it. I bought 2 of them because of it being cheap (on sale for $70, regular $99) and narrow so it worked well in tight spaces. I don't think that it will make a good high back booster because the belt guide isn't adjustable. It would be fine as a no back booster though. If you are wanting a seat that you can use as a high back booster too, you would be better off getting something like a Nautilus or Frontier.