A1: if you select no bootloader, then the installer won't install one (which is quite clear). That means: if you have already an operating system (thus a bootloader too) installed, it won't get overwritten. The other side is that you will have to manually configure that other bootloader (be it Windows bootloader or other Linux' bootloader or some other bootloader) manually and add a new entry for your newly installed operating system to be able to boot it, unless you create a boot diskette during setup.
A2: there are probably numerous things you can pass as parameters, like force VESA graphics mode ("safe graphics") and so on. They're probably easily found using Google; usually if you want to boot your Linux into a single user mode, i.e. into a mode where only one user is active and is (you) logged in as root without asking passwords (a "maintenance mode"), you can pass a parameter single
to the kernel. This is one of the things why you should set a bootloader password; it prevents people from booting your machine into single user mode without passwords and resetting root user password (or do anything else superuser can do). Oh, and you definitely don't want to add that as the default option; just a note.
A3: I think it's just up to the installer. I can't speak for every single bootloader out there, but LILO and GRUB do provide a "default" option that tells which entry to boot if none is selected in a given time. For example with GRUB's config you can define "default 0" to set the default entry to be the first in the list; "default 1" would be the second and so on. Leaving the option off means usually the same as "default 0". It also supports, as far as I know, to set the last selected as the default one. To set this after the installation, alter /etc/lilo.conf
(and in LILO's case after editing the file -- READ MAN PAGE FIRST -- run the command lilo
as root to re-install LILO for the changes to take effect; without running it your old settings will be used) or if you use GRUB, alter /boot/grub/menu.lst
(the current GRUB Legacy config file; in this case no need to run grub or do anything further, GRUB reads the changes during boot from the file).
A4: MBR, Master Boot Record, means the first 512 bytes of the harddisk. Those 512 bytes are a "reserved area" that is referred to as MBR, and is meant for a bootloader. Windows, as far as I know, always installs it's bootloader there, and that's what I suggest you do with your Linux bootloaders too. Actually the first 446 bytes from the beginning of the disk contain the actual bootloader information (again, as far as I know) and the rest up to 512 hold the partition table information; after this the following bytes contain other information such as maybe a copy of the partition table, the partitions and actual data. "First sector of boot partition" means what it says; instead of the first 512 bytes of the whole harddisk the bootloader data is written to the first bytes of the partition used to boot the operating system (in a Linux installation, /boot partition if it's separate, or if you only have root partition, then it's that). So MBR is for the same thing as the first sector of your boot partition, but of the whole harddisk instead of a partition. MBR is used in the first hand, I think, because it's a good habit to have one place where your BIOS knows to look for a bootloader; no need to guess where it is, if it's said MBR is the place for bootloaders
Hope these answers shed some light. Others have surely more to say.