Nexus in Delaware
There are basically two questions that need to be answered, each related to a different type of tax:
1. Federal Income tax: this tax is paid on income produced by the company in the US. If you are producing such income you would be liable to pay income tax, regardless of whether you have any physical presence in the US or not. So having a virtual office in Delaware or not is irrelevant to this particular type of tax.
2. Sales Tax: the question of nexus is actually more related to sales tax, in other words having a nexus in a given state would require you to collect sales tax on retail sales to residents of that state. Again, in your case it is irrelevant, because Delaware has no sales tax, and as far as I understand you don't sell anything in the US anyway.