If you are suspending someone there is nothing 'impolite' about using the word suspension. It's simply the correct word for the action being undertaken and it is used in this situation by companies. You shouldn't attempt to use another word as that would simply be confusing and, frankly, silly. You need to be as clear and business-like as possible, and just state the facts. Your letter is just hints and vague comments. They need to be told that they are suspended, the date they are suspended from, and why they are being suspended - 'you know why' is simply not good enough. They also need to be told how long the suspension is in place for and how they can appeal against the decision/what investigation is being undertaken, etc. They also need to be informed about what is happening with their salary during the suspension. There may be other local legal requirements. If I got a letter like yours I'd a) be very confused and b) sue the company once I'd worked out what you meant.
Are you clear about what a suspension actually is? When someone is suspended they are still employed by the company, usually still get paid, but they don't come into work, and it's normally for some sort of disciplinary or investigation purpose.
It has nothing to do with being asked to work without a manager.