His argument on the Welsh influence is based on the use of what he calls the meaningless "do", in particular in questions and the negative.

Seems a mighty slim peg to hang such a pronouncement on. I'd have to look at his argument and then at Welsh. Any idea what other historical linguists have to say about it?

Is the ablaut used to mark tense in any non-Germanic languages?

Yes, but not only. It was originally something to do with the change from a tone system to a stress system of accent. You can see traces of ablaut in IE languages such as latin, Greek, and Sanskrit.

And, come to think of it, what were those Semites doing mingling with the Germans?

Ceci n'est pas un seing.