I am not a sedevacantist, but would like to know what the informed opinion is on whether someone can attend a Mass and receive Holy Communion in a sedevacantist chapel.

Everyone should note that it was precisely for this kind of thing that Nado was banned in the first place, and not for sedevacantism.  Just for the record ... lest people think that Matthew bans sedevacantists for that reason alone.

I can picture you 5 years from now, sitting in your old-man rocking chair, at every rock forward, uttering, "Nado..."
Sounds pretty sick to me.

You act as if it's a given that "trad-cuмenism" is a bad thing.

Are you willing to first define "trad-cuмenism", and then explain to us why "trad-cuмenism" is as bad as the false ecuмenism of the Conciliar Church?

I might have serious discussions, but at my semi-mature age I learned long ago who to discuss with, and who to ignore. You fit into the latter category.

Catechism of the Council of Trent page 399 and 400.  (30)  The Apostle also teaches that they are entitled to obedience: “Obey our prelates, and be subject to them; for they watch as being to render an account of your souls.” (31) Nay more, Christ himself commands obedience even to wicked pastors: ” Upon the chair of Moses have sitten the Scribes and Pharisees: all things, therefore, whatsoever they shall say to you, observe ye and do ye: but according to their works do ye not, for they say and do not.” (32)
The Passion of Our Lord completely obliterates modern sedevacantism because Jesus Himself recognized wicked Church authority and subjected both His Humanity and Divinity to it.  Jesus willingly subjected Himself to some of the most despicable and heretical church authorities in history.  Our Lord did not demand that Caiaphas resign before allowing him to manhandle Him. Some will respond: “wickedness isn’t heresy”.  That is a ridiculous notion because heresy IS wickedness.  No one can prove the rulers of Christ's day weren’t heretics.  Our Lord didn't usurp Annas’ authority because of the man’s corruption, or deviancy from the truth, or heresy. Rather, Our Lord Jesus Christ went as a Lamb to the slaughter. Some say, 'that was Jesus', 'he was God, it doesn’t count'. Sorry, that won’t do because Jesus also never admonished Mary for her silence in the face of heretics run amok.  Jesus never said, “do not recognize their authority, Woman. They do not officially hold office or power over us, let us wait for a true leader with the Faith.” Rather, Our Lord even takes special care to explain to the world that Pilate’s authority (over Him) came from God above. In order to honor His Father, Jesus submitted to a variety of authorities, both religious and secular, not just in day to day matters, but even unto death.  And Mary imitated Her Son.  As our premiere role models, Jesus and Mary show by example that all Catholics should do as they did, knowing Our Lord and Our Lady were properly obedient without committing a single sin.