We purchased a 5-year old home with walk-in marble tile shower that has
developed small cracks between the tile walls and tile floor and also
vertically between the side and back walls.

An important question to ask is why are they cracking? Are the tiles
properly mounted on backerboard or did someone just slap them up onto
drywall? You can seal all you like but if the wall behind them is bad then
you're just postponing the inevitable.

Re-grouting would probably be better. They make tools to remove the old
grout. Scrape it out and apply fresh grout. If they joints aren't tiny
then it shouldn't be all that difficult.