1. All business activities in Malaysia must be done with a valid business registration. Sell roadside nasi lemak, or give tuition, or supply small parts to company, or just drop-shipping online must have business registration. I make no comment on whether it is enforced. This is the law under Registration of Business Act 1956 Sec 5(1), sec 12(1)(a). Don't listen to "opinions" for this, BUT you have the information, and you decide based on the information and then live with it.

2-9. The business is you and you are the business. No matter which account the money is deposited, it is all counted during tax time.
Yes, capex and expenditure like buying materials can claim as business expense.
No, cannot have a trade name later. From what I understand must close one and open the other. (I may be wrong though)
If you have small business, but always losses, (my friend in financial industry told me, I cannot say true or not) the system will flag for audit. Must have small profit at least, otherwise why you running the side business if always losses?
Yes, financial statements not required to be submitted, but you still need internal accounts so you can fill in the tax form. Must keep receipts for 7 years. Must be able to prove the figures if/when audit.

Source - Own self is employee, but also have sole-prop business since 1996 and fill in tax form every year. This is what I have been doing as I understand it, but maybe wrong also so just be aware.

If you don't have a business registration, then you will not be able to open a bank account. You could receive money in your personal bank account, but the bank may flag it if there are many deposits all the time. Think of it this way : Your salary is RM 5,000 and you have side business income of RM 500. You can treat it as if you have a salary of RM 5,500 and spend it any way you want.

So you'll be taxed as if you have a salary of RM 5,500, just that it is summed up as "salary" and "business income".