Just found this thread. I will try to answer a few of these questions:
> ... 1. Do WR-parishes celebrate holy Mass on the feriae of Lent?
Some do and some do not. Only the old-calendarists have an actual prohibition on that, in Western Rite. In the RWRV, both practices (full Mass on Lenten weekday, and Presanctified Mass) are known and approved.
> ... 2. Do WR-priests use a corporal of linnen or a byzantine antimension?
This has been handled in different ways, but there is no reason that antimins cannot "double" as the next-to-topmost corporal. There is no strict requirement that altar cloths be linen, since silk was often used for them in the middle ages.
> ... If they use a corporal of the Latin tradition, is it considered necessary that the eucharistic sacrifice is offered over an altar (of stone), containing relicts of martyrs and being blessed by a bishop?
That is normative, but the antimins can serve the function of a "superaltare" in the Western Rite, permitting the celebration of Mass without an altar of stone, etc.
> ... 3. Is the sacrament of consignation/confirmation (by laying on the hands and signing the cross on the forehaed) reserved to a bishop?
Generally speaking, it is not. Most WR Orthodox follow the (minority) Western practice in that regard, a practice permitted specifically by St. Gregory the Dialogist and found in parts of Italy prior to the Schism of Rome in 1054.
> ... If this is the case, is the (first) postbaptismal unction considered as participation in the royal, prophetic and priestly dignity? How is it performed (prayer and gesture)?
Yes. The details are in the service books approved.
> ... Or is this unction considered as sacrament of confirmation (chrismation) like the byzantine tradition does?
Beware the premise that the Byz. Rite "considers" anything in the Western Rite as having a particular meaning. The Byz. Rite addresses only the Byz. Rite. I can say that the anointing in the WR which you refer to, is not considered the sacrament of Holy Chrismation, which is made even clearer by the fact that it is followed, later in the service, by a complete Rite of Chrismation.
> .. Is the hl. chrisma (myron) the same, that is used by the fellow ER-parishes, blessed by the first hierarch (patriarch)?
> ... Is it thinkable that - if one day an ordinary WR-Dioecesan Bishop may be consecrated - that he will obtain facultas for blessing the chrisma himself?
I suppose it is thinkable. It's a bridge which has not been crossed, so to speak.
> ... 4. Is the Apostolic creed in use? if yes, only in the Liturgy of the hours or even in the Divine Liturgy?
Yes, only in the Divine Office, never at Mass.
> ... 5. Where is the Creed placed in the Divine Liturgy (holy Mass)? Always after Gospel (resp. the homily) or also after the dypticha (commemoration of the names), if said?
You may consult the approved texts for the answer to this question. It comes "earlier" in the service, in the WR forms.
> ... 6. Which formulas of the Epiclesis of the Holy Ghost in the Anaphora (Canon of the Mass) are in use?
There are two of which I'm aware. The first is simply the epiclesis from the Liturgy of St. Chrysostom. The second is a non-recurring feast-day prayer from the Gallican service books, for the feast of St. Peter's Throne, which has been made to serve as a recurring epiclesis prayer in the St. Petroc use.
> ...If only this is the case, why is the old Latin/Roman Holy-Ghost-Epiclesis, as it is whitnessed by Pope Gelasius in his letter to Elpidius and litteral in Oldspanish as well as in Beneventians and some other manuscripts, not (yet) restored?
No explicit reference to such a prayer exists in the document record. If the presence of such a prayer is inferred (and some do indeed make this inference), then one needs to come up with a text for it. And the only text known to have been used in that function, is found in the Byz. Liturgy (or, possibly, other Eastern rite services). Thus the phenomenon of the Byzantine epiclesis in the Western Rite Canon Missae.
Regarding Corpus Christi, many WR clergy do not observe this late Roman Catholic custom, and many do observe it. Exactly how they observe it, is something I don't know much about.