Go to a valid Mass and receive the valid sacraments whenever you can, for as long as you can, and otherwise keep your involvement to a minimum. Don't get to "Parishy" or identify to heavily with this or that group or priest and keep it all at arms length or you will run into trouble at some point.
We are not living in a normal time or situation and trying to act as though we were and normalize your religious life is a folly. It cannot be done at present. Use what is available and to your soul's benefit and eschew the rest.
We are Catholics plain and simple, and we do not need to belong to something to save our souls. We belong to Christ...it is enough.
I mostly agree, but I would like to add that SOME people are detached/objective enough to be nice, social, discuss the Faith, etc. over donuts with fellow parishioners, and STILL be ready to bolt if the group ever develops serious problems (compromise with modernism, cult behavior, evil tactics from the priests or superiors, etc.)
Ideally, that is how everyone would be.
I hate to recommend that everyone be callous and rude, not talking to any of their fellow Catholics, because they will get "emotionally attached" to the group and the parish.
How are traditional Catholics supposed to keep the faith, without human interaction, support, and friends?
Being sociable is part of being a Catholic, it is expected as is good behavior, courtesy, and kindness. Being callous or rude was not at all suggested and the point is that if you allow yourself to become involved, you are risking losing your objectivity and thus becoming susceptible to deeper involvement that you had wished and all of the other things which you have mentioned.
We are living in grave times and above all being deadly serious about your salvation and that of your family is what will determine what is to come. There is no longer a margin for error, and foolishness such as that letter and other SSPX and resistance antics are a waste of time and a waste of God's gifts.
And they are the result of becoming to involved and losing both perspective, objectivity, and then the truth.