It sounds like you may have some misconceptions about the intentions behind bustabit's offsite system. Our system is designed to allow investors to reduce their counterparty risk and increase their liquidity. Leverage is used to accomplish that, but allowing investors to choose a static leverage and maintaining that is not its goal.
Assuming you don't misrepresent your offsite investment, you will always risk 0.75 % of your total investment per bet. Whether that is "safe" is debatable–is investing in a casino's bankroll really ever "safe"?–but it is the optimal risk for maximum bankroll growth (see Kelly criterion). That means that leverage will decrease as your the onsite portion of your investment grows and increase as the onsite portion decreases. This is not a flaw, but the correct behavior and by design.
Regardless, automatically reducing your leverage as your onsite portion decreases has the same risks as being margin-called: In both cases your stake will decrease and you won't be able to benefit from an upswing without manual intervention. So I don't fully understand why you consider static leverage to be safer. If you want to be "really safe" and avoid the risk of being margin called entirely then depositing your entire investment is your only option.
The problem is that it's not realistic to be able to monitor the investment all the time and therefore to deposit that offsite money if needed. So if the casino keeps losing, your "leverage" grows at a higher than the desired rate, so potentially you could lose all your onsite investment (as the casino assumes you still have the same amount offsite which you could deposit). It may be an extremely unlikely event, but eventually it will happen.