when you are not an expert, then i suggest to add the plop boot manager to the grub2 menu.
I further want to know if hard disk install using GRUB2 is possible?
yes, when grub2 is installed to sda2
I want to be able to use plop even if I reinstall operating system on sda2 ( plpbt.bin is in sda2).
i assume, you mean, when during the install something goes wrong, then you are not able to boot again from usb. what do you have on sda1? you can copy plpbt.bin to that partition and in emergency case run it from there with the grub command line (when the command line still works). but thats for experts.
Am I right in thinking that if Plop Boot Manager is installed on the hard disk then overwriting Grub won't delete Plop Boot Manager and it will still be present on the hard disk?
when grub is installed to sda (=mbr) then the plop boot manager is still on the hard disk, but will not start until you recreate the plop boot manager loading routine in the mbr with the intall program. but this will remove grub, so you have to be sure that grub is on sda2 before you do that.
best regards
elmar