Yes, the Lamb was slain. But when John first sees it, it was standing as if slain. The standing clearly refers to a Lamb that was living. But nevertheless, it had the appearance of the sacrifice. Clearly, the death was present in heaven, although the Lamb can never die again.
That’s why the Eternal High Priest has to have an Eternal Sacrifice, otherwise, he would have retired on Calvary, and could not be a priest forever.
Moreover, it’s past tense only in the English. In the Greek, the verb for slain is in the perfect passive. That’s why Revelation 5:6 is somewhat awkwardly worded: standing as if slain. He is alive, but the “slain” condition remains even to the present.
Baptism is another re-applied sacrament - unlike the Eucharist, it isn’t a sacrifice; but it is like it in being one sacrament, which is applied time & time again. In no way does that “re-baptise” Christ - neither does this re-crucify Him: He has conquered & abolished death, & has been Glorified at His Father’s Hand; nothing is “done to” Him; but He acts in us through His Spirit.
It is as certain as anything could be, that the Sacrifice of the Cross is unique, all-sufficient, all-efficacious, expiatory, propitiatory - so the Eucharist is as well; they are the same thing, identical in all values, different only in manner. One was bloody & agonising, the other is bloodless, being the setting-forth & application & exhibition of that perfect atoning sacrifice by the power of the Holy Spirit; Who takes what is Christ’s, & makes it present wherever the Church of Christ is present. And He does, through human beings, just as He evangelises & preaches through human beings.
As for the offering - it has been offered for all time: what men offer, is the sacramental, therefore efficacious, sign of that unique & perfect & unrepeatable offering: the sacramental sign signifies visibly what it brings about invisibly. In this Sacrament, visible food & drink are offered that they may become the Food & Drink of the inner man, so that we may become the One Whom under these signs we eat. The Christian priesthood is not like the Levitical - they have only that One Offering to bring, by way of application, in Christ the Priest; they can offer nothing else, because they have nothing else to offer; & no other Offerer but Him