Hello ForWhomTheBoneBones,

you asked:

How might (English king Richard I) have been treated?

The German emperor Henry VI as well as duke Leopold V of Austria had good reason to tread carefully. As a crusader, Richard was under the protection of the Church and both were bounf by oath and papal edict not o harm him. Furthermore, both knew that eventually they had to release Richard alive - not only to collect the ransom, but also to avoid being excommunicated - which was what promptly happened to Leopold. Before Richard was transferred from Leopold's "care" at Dürnstein to Henry's, duke and emperor met at Würzburg to negotiate a contract over the English king. In that contract, Henry promised Leopold the latter's share of the ransom even in the event that Richard should die while in the emperor's custody in Ochsenfurt. It seems that Richard didn't fare too badly there:

When, hasting form England on the news of Richard being alive but imprisoned, the abbots of Boxley and Robertsbridge met Richard, they found him healthy and in good spirits. Nor was he isolated; to the contrary, Richard was allowed to transact the business of his kingdom, friends and envoys could come and go unhindered. Richard's mother, Queen Eleanor, travelled to Richard in January 1194. Richard had even his favourite falcons brought over. Thus he arranged to have his rebellious brother John confined and a truce between the factions of the on-again-off-again civil war to be arranged. Richard secured the support of German princes such as the duke of Saxony as well as of the German Clergy to avoid being handed over to Philip II of France, who dreamed of conquering England. In fact, Richard was even able to arrange form afar the eletion of the archbishop of Canterbury and, during his stay at the imperial court in Worms, to write poems.

On his way back to England after his release, Richard spent three days in Cologne with his most important German ally and friend, archbishop Adolf, where he was sumptously entertained while staying the archiepiscopal palace.

What precautions were taken to ensure that his ransom arrived in one piece?

The ransom was transported by German envoys and at king Richard's peril. I have no primary or secondary sources as to how this particular cargo was transported. However, the usual seagoing vessel at the time in nortwest Europe was the cog. These vessels sailed both the sea as well as rivers and could be fitted with defensive fore- and aft-castles, which could house archers[3].

Sources:

[1] Joseph P. Huffman: The Social Politics of Medieval Diplomacy: Anglo-German Relations (1066-1307). University of Michigan Press: Ann Arbor, 2000 - ISBN 9780472110616

[2] Frank McLynn: Lionheart and Lackland: King Richard, King John and the Wars of Conquest. Random House: London 2006 - ISBN 9780712694179

[3] Ruth A Johnston: All Things Medieval: An Encyclopedia of the Medieval World. ABC-CLIO: Sta Barbara 2011 - ISBN 9780313264624