Congrats on your new van, the '11 Odyssey is awesome.
Although I can understand the temptation to do so, definitely don't use lower LATCH anchors and the vehicle seatbelt to install at the same time. Neither the carseat manufacturer nor the vehicle manufacturer allow this, either because it hasn't been tested and approved to do so, or it has been tested and found to adversely affect crash performance in some way.
Definitely use the vehicle seatbelt for installing in the Ody at 40 lbs. and up as Hondas have a 40 lb. child weight limit for using the lower LATCH anchors. It's also always allowable to use the seatbelt to install for kids under 40 lbs. in a Honda if that achieves a successful installation with less than 1 inch of movement at the carseat's belt path, but don't use both the seatbelt and lower LATCH anchor installation methods simultaneously.
While I haven't done installations in the '11 Odyssey, I have installed Regents and Frontiers in other vehicles and have never failed to successfully do so. Sometimes it takes some practice, trial and error, and sometimes it takes a second set of hands to get it done.
The Frontier allows you to use the short or the long belt path seatbelt installation method at any child weight so you're not limited to only using the short belt path method for the Frontier with kids over 40 lbs. In my experience, the long belt path seatbelt installation has usually helped to get a more solid install of the Frontier and the Regent. That said, though, if your Regent is a pre-advisory Regent made before approx June 18, 2007, then it allows you to choose between the short belt path and long belt path installation seatbelt methods regardless of the child's weight. But if your Regent was made on or after June 18, 2007, then it's a post-advisory Regent with installation method requirements that vary depending on the child's weight, and you need to use the short belt path seatbelt installation method at 40 lbs. and up for a post-advisory Regent.
I would try the long belt path seatbelt installation method with your Frontier, and also with your Regent if your Regent is a pre-advisory model, and also experiment with different seating positions in the van to see if you can do successful seatbelt installs with less than 1 inch of movement at the carseat's belt path for both carseats. A second set of hands can be very helpful when doing a long belt path installation if you can enlist a helper to hold the vehicle seatbelt almost fully extended from the shoulder belt retractor but just shy of engaging the retractor's locking mechanism while you're threading the seatbelt through the carseat's long belt path, buckling the seatbelt, then locking the retractor and tightening up the installation. Sometimes reclining the vehicle seatback slightly and then raising it back up while tightening the installation can also help achieve a more solid installation. Hopefully these tips will help you out, and congrats again on your new wheels.