It's not a "User Changable" battery, however many largely popular devices are not. I've been inside the 740 and it would not be major surgery, however it would void your warranty. This is a circuit protected Li-Po battery, it should not need replacing anytime soon.
As for the AHRS, the 740 will not be available with an internal AHRS. Though it will be compatible with many popular AHRS devices like the iLevil, Clarity SV, SkyGuardTWX and more. It's really more practical to keep these high cost modules like AHRS, ADSB, XM seperate.
Most would hate to want to upgrade and have to repay for those high cost items again, because they're integrated. It's also needed to have the AHRS permanently mounted in a Fixed / Level location, this would not work for yokes, kneeboards, handheld, etc. A quality aviation suitable AHRS is not cheap and would add sibstancially to the retail, we've tested many a AHRS boards. Good questions.