Hi Hasan,
1) Since summary accounts take a lot of burden on the system resources, could you please tell me how to delete or disable any summary template no longer needed? Or, could summary template be modified in terms of Details or Total, once in use? Thank you.
3) What is the choice of TOTAL instead of DETAIL when creating a summary template?
2) Could Rollup Groups be deleted or disabled?
Thank you.

Simply go to the Summary Template screen and delete the template. Summary accounts would automatically be deleted.
Go to the Rollup Group window and try deleting a rollup group. You will get your answer to question 2.
I have explained point 3 in detail in summary account videos.
Here is an example

Combinations Used:
01-1000-00
01-1100-00
02-1000-00
02-1100-00

If you want to view account balances by companies, you can create a template:

T-D-00

This template would result in these accounts:

T-1100-00
T-1000-00

A parent value T must exist in your company value set. 'T' is assumed to contain all values as its children. Ranges does not need to be defined for T.

What would this result in?

D-T-00

This template would result in these useless accounts:

01-T-00
02-T-00

01-T-00 would display sum of all accounts for company 01, which should be zero if your books are balanced.

02-T-00 would display sum of all accounts for company 02, which should be zero if your books are balanced.

A parent value T must exist in your account value set. 'T' is assumed to contain all values as its children.

Rollup groups could be used instead of T as well, as explained in videos.

Value T in summary account is equivalent to a rollup group T (which does not need to exist) containing a parent value T (which should exist) containing all detailed values (no ranges need to be defined) as its children.

Thank you.