for future travel there will be no consequences, unless you try to enter illegally into Germany and then try to legalize your status. However, in case of you will be involved in a similar incident again, the fine (the amount of daily rates) will be considerably higher. When reaching 90 daily rates the criminal offence will be registered in a way that may appear in normal background checks. Lower fees will only play a role in very special occasions (job in a German authority, etc.).
Yes. 5 Euros is the lowest amount possible, equivalent to a monthly salary of 150 Euros. As you said there are several witnesses against you, I would accept the Strafbefehl.
The fine depends on your salary after tax. 30 daily rates = salary of 1 month
If you accept, the 40 daily rates will be internally registered in case you may commit a similar criminal offence. In the latter case the fine will be higher.
When appealing to the Strafbefehl, the issue will go to court. Usually the fine will be higher if you are not held unguilty. In your case, I just won't recommend it.
An isolated appeal against the daily rates (for example the amount) is possible, but does not make sense in your case.
I would forget the whole issue.