It's ICKSP so 99% not a real priest. He should still ask to make sure though.

If he's not valid God will still provide you enough graces through the rosary, perfect contrition, etc. but going to confession is still a must.

Just do it. You have nothing to lose and everything to gain. The whole point is that doubtful orders are to be considered as invalid for even if the chance is small they are invalid the risk involved is enormous.

The risk/benefit ratio is wildly in favour of attending the byzantine rite.

Indeed, why would you stand up a "controlled opposition" to keep alive a movement that would have pretty much died out by now without him?  Had there been no +Lefebvre, no SSPX, we'd be witnessing the dying off of the older pre-Vatican II Traditional priests (Father Leo Carley near me is one of the last, in his late 80s now but still going strong) ... apart from PERHAPS the +Thuc line, but God knows whether +Thuc would have had the courage to do what he did had Archbishop Lefebvre not set something of a precedent in resisting the Modernists.

But ... I actually don't believe he committed any kind of "grave error" on the Pope question. He made it clear that he believed that this degree of destruction was not possible given the guidance of the Holy Spirit over the Church and the Papacy, stated repeatedly that the Conciliar Church does not have the Marks of the Catholic Church.  He simply felt that he lacked the certainty of faith (and was not incorrect) to judge the Holy See vacant ... barring some other possible explanation.  So he merely prescinded from the ultimate conclusion due to lack of absolute certainty.  His mistake was to not articulate this more emphatically, with the result tha many of his followers amplified his position into the dogmatic R&R we see today, something which in the final analysis as closely akin to Old Catholicism.

I believe that +Lefebvres serious error was on the theological foundation for all the Vatican II errors, EENS dogma and the resulting ecclesiology.  His articulation of EENS leads inexorably to the V2 ecclesiology as its logical conclusion, and all the V2 errors flow from that ecclesiology.  So he misfired on that, and that goes to show how deep the rot had gotten on that particular issue.

Agreed.  There's no doubt about the validity of the Eastern Rite Sacraments ... apart from an occasional straggler who switches over from the Novus Ordo (and then of course they don't conditionally re-ordain them).  You don't see that often with the Ukrainians.  Even the Maronite Sacraments are certainly valid, although they've definitely modernized the externals of their Liturgy (more vernacular, lay readers, even altar girls, turning the altar around, pushing tabernacle off to the side ... although the basic forms haven't changed, and they even say the words of Consecration in Aramaic).  Dimond Brothers had no issues receiving the Sacraments from Eastern Rite priests.

Now, you may encounter slightly bit more Modernism in the Catholic Eastern Rites (moreso among Maronite and Byantine, less so among Ukrainian), as conservative Novus Ordites tends to flock to the Indult/Motu ... but that's the tradeoff for valid Sacraments.  Also, there's no question of Communion in the Hand because the Byzatine families of Rites distribute Holy Communion from a spoon that contains Both Species while the Maronites use intincture (dipping the Bread Species into the Wine Species) ... also rendering Communion in the Hand impossible.