Methodists use grape juice due to the long association of Methodism with the Temperance Movement, especially in the 19th century. In fact, pasteurized “grape juice” was invented specifically for use in Methodist communion. It was developed by a Methodist deacon looking to make “unfermented wine” for use in his local church.
His name was Thomas Welch. As in “Welch’s,” purveyors of fine fruit juice drinks, and a company you may have heard of. 
Not even for gluten allergies. There was a big kerfluffle a few years ago about that. The Church eventually approved low-gluten hosts, but only for those who suffer from celiac disease, and even that can be problematic for them. Gluten-free is forbidden, because it won’t be wheaten.
Celiac sufferers can, however, take communion by only taking the wine (obviously from a chalice other than the priest’s, which has a piece of host in it from the Consecration), but that presupposes that the sacrament is being given in both species, and I don’t know if that’s very common (not having been to a Catholic mass since my father’s death in 2001).
In the Methodist church when I was growing up, they used leavened bread.
Does the communion wafer have to be made from flour and water only (in churches that insist on unleavened bread), or can other things (like salt) be included as well? If it is flour and water only, you’ve got my sympathies- I’ve been eating lots of matzo for the past week.
Here’s a basic but decently detailed list of mortal sins.
Yeah you have to use your brains a bit, but the list is pretty obvious Bad Things. Now, on the other hand, I really doubt that people who have masturbated think of it as a MORTAL sin. 
It is given in both species the majority of the time in my experience. Also it wouldn’t be difficult at all to do a chalice just for that one person even if the rest of the people were only receiving under once species.