The UK have started the 2-year clock running on Article 50, and IIRC preliminary negotiations would have been underway before June which I can only assume now can't happen as we won't have a government so we can't have a formal negotiating position.
Everything about the UK's official negotiating position will go up in dust until such time as May is re-installed as PM. Presumably, the negotiating teams on both sides will have to sit on their hands until then, and if the Tories lose their majority there will be a massive delay while we sort that mess out.
Cut it anyway you like, May's argument is completely indefensible. Yes, elections are a normal thing in UK politics, but dissolving a government with a functioning majority weeks before the EU negotiations start is not normal. This is Tory party political manoeuvring and nothing more.
I can only assume that 'now was not the time' for a Scottish referendum because the reality is that the Tories have had this GE planned for months. They were probably hoping that Labour would oppose Article 50 in the Commons, but went to Plan B when that didn't happen.