Dear Querist,
A. Heading '998391' is for 'Specialty design services including interior design, fashion design, industrial design and other specialty design services' whereas subject register's services are 'actual execution using material & labor'. And hence, subject services would fall under Section 9954.
B. So as to answer your second question, it is my presumptive understanding that services provided by the tax-payer falls under works-contract as defined u/s 2(119) of the CGST Act, 2017.
B. Here, I hold a view that such tax-payer is also compulsorily liable to take registration in Mumbai (Maharashtra) under GST.
B2. For reasons behind my view about mandatory requirement of taking registration at Mumbai (the actual site of work) in situation under discussion here, one may look into my multiple posts under Issue-ID: 118425bearing subject-lineas 'place of supply, Goods and Services Tax - GST'
B3. I am aware that many of such type of tax-payers avoid taking such registration in other states. But, avoiding such registration in other states (where actual site of work lies) can be risky & litigation-prone from multiple angles in my humble view.
C. If subject tax-payer is in-turn receiving services from local vendors from Mumbai / Maharashtra where place of supply is 'Mumbai / Maharashtra' (& not Bangalore / Karnataka) because of certain situations covered under Section 12 of the IGST Act, 2017 (For example: situations falling sub-section (3)) and based on my professional experience, this happens very often in these line of businesses, not taking registration at Mumbai will also result into loss of ITC for such tax-payers.
D. From the legal requirements as explained in Para B above & business requirement as explained in Para C above, it is better to take registration in other states (where actual site of work lies) in this line of business.
D1. Compliance costs of additional registration/s are far less that 'risk, litigation-costs and loss/es in ITC' in my humble view.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion.