There is simply no way to have Quicken "do your accounting for you", PARTICULARLY when it comes to Investment Accounts. Go to Edit > Preferences > Downloaded transactions and make sure that "Automatically add to investment transaction list" is not selected. That will allow you to tell the Placeholder suggestion to go away.
As a general statement, if you want accuracy in your Account, you should never accept a placeholder. Frequently Quicken is simply wrong here, and if it's not then you have a problem in your Investment Account that you need to solve.
The automatically generated Balancing Cash Adjustment generally means that there are one or more Placeholders for the affected security at a later date than the transaction you are trying to enter.
You must delete the Placeholder(s) before Quicken will let you successfully enter any transaction for the problem security that affects the cash balance. Once you have deleted the Placeholder(s), you can delete the cash adjustments.
Are you speaking to variances in the cost basis, or market values? You say you want to make an adjustment without affecting net worth, so I have to assume it's a cost basis difference, right? Are your cost basis values higher or lower than that shown by the financial institutions, or maybe both?