Can a pastor who is leaving the church for a position as a full-time foreign missionary (the church will contribute missionary support as the sending church) receive retirement compensation for past services by declaration of the church? What would be the taxability of such an arrangement? Does it matter how long is is paid or whether it is in a lump sum or a monthly obligation?
Probably the easiest way for the retirement contributions to be made as implied in this question is to have the church send support to the former pastor's account with his mission agency employer. Then the pastor may make elective deferrals into an Internal Revenue Code 403(b) plan. He can likely contribute in excess of $20,000 per year (IRS Publication 571 describes the limits).
Can the church continue to pay for health insurance as a tax-free fringe if the senior pastor is no longer an employee but simply a missionary from the church?
Similar to my answer for Question 1, the pastor's mission agency employer can provide health coverage using funds supplied by supporting churches, including the pastor's former congregation.
Can the church compensate the outgoing pastor with severance pay before he goes to the field? Would that be taxable like his pastoral salary?
Yes, the church can provide severance pay, but it will be taxable as any other compensation. To mitigate the tax costs, standard ministerial tax law benefits should be pursued (e.g., housing allowance designation, elective deferrals into IRC 403(b) retirement plan).