use of Noto fonts

Hi, I’m wiring this because of the use of Noto fonts.
I know that it published under the SIL OFL 1.1 and I have a few questions about the copyrights.

Recently, client wants to merge between specific fonts (they hold the copyright), and Noto fonts to make a products with embedded fonts are in it.

When they merge, can Noto fonts change its sizes or weights? And if so, how much could it be?

They are going to contain the SIL OFL 1.1 notice in their products, but they are not quite sure about how much portion of source code should they distribute, because they merged font that has a copyright.
Is it compulsory to distribute all source code? Or just related parts with Noto fonts?

In this situation, can this merged fonts protected under the copyright laws? As in, does the Client (copyright holder) still have a same copyright of this new modified font? (eg. someone hack the product and use their fonts) Just be sure that they are going to sell the electronic products not the font software itself.

JaKyung Koo

(answered in duplicated post)