I see you are about to leave and I hope you have gotten your plans in order by now but I hope I can also still give you a little info that may help you during your stay.
In regards to Mardi Gras, in my opinion it doesn't seem to have as much of an impact as Spring Break does which impacts both parks. You ARE going to be there on the tail end of the Spring Break period so though the lines will be long I do not believe they will be as long as they were the last several weeks. Also depending on who is performing that night will have an impact on the crowd. The whole Mardi Gras thing really doesn't get going until the late afternoon / early evening and if you are not into the parade or the concert it can actually work to your advantage, though I would recommend the parade it is a lot of fun. We have attended Mardi Gras there many times over the last several years (including just several weeks ago) and in the past some of the wait times for the attractions during the parade and the concert can be relatively short.
The last time we were there BOTH parks had early entry for on-site hotel guest but ONLY for the Harry Potter rides. From my experience if you are going to use this perk you should arrive as close to the early opening as possible. We went one time and got there about 1/2 hour early and the line line for regular park / non-hotel guest was already quite long. Well as we enter and on our way back to Forbidden Journey a flood of people came up from behind us, quite quickly I might add. We had not been in the park but for more than a few minutes and could still see the entry gates. Well I guess they decided to start letting the non-hotel guest in early since not that many on-site hotel guest were entering. Not sure if this is common practice or just our luck that day but regardless this did not sit too well with us but what were we going to do.
You mentioned you are staying at Cabana Bay. I must say I do like this resort (2nd favorite to Hard Rock) and though I understand WHY they do not offer Express Passes there I wish they would at least offer discounted passes for being a resort guest but that is just my rant (that could actually make it my favorite though.... maybe). If you don't mind walking I would suggest taking the walking path to the parks. It is long (about 1 mile) but we found that it can be a nice quiet stroll after a busy day or evening. We also we challenged our kids to the the amount of time it takes to walk vs the shuttle bus. It can be about the same. My wife and I split off from our son and daughter at CityWalk, we walked they took the shuttle. They said they waited an average amount of time for the shuttle bus when they got to the bus stop (they never walk always take the shuttle bus). We didn't walk any faster than we normally would. We made an agreement that when either group arrived at the resort to call if the other hadn't already. Just as we were coming in the resort from the gate they called us and said they were just getting off the shuttle bus. Now of course depending on where you are staying in the hotel would depend who won this challenge. In our case it was us. The path comes in by the courtyard units, where we were staying, the shuttle bus drops you off by the lobby which would be by the tower units. Any way take some time to enjoy this resort during your stay you will be glad you did. If it's warm enough you also need to float a couple of laps in the lazy river, by the way you can use your floats much cheaper than their's unless you are looking for a souviener.
I hope I didn't go on too long and you find this at least a little useful even as late as it is in your planning.
More than anything just go with the flow and enjoy your stay!
