Historically, the battlefield usage for matchlock muskets was not designed for individual fire, as you noticed, the process is slow and rather difficult. The Dutch system used multiple ranks to maximise reloading, so called because it was made famous during the reforms of the Dutch army during the Eighty Years War, and its development by Swedish and German usage during the Thirty Years War (usually referred to now as the Swedish system). There were variations in depth and width, but fundamentally, in the early Dutch system, musketeers operated in ranks of ten, when the front rank fired, it retired through the files of the rear ranks, and the second rank (now the first rank) fired, and then retired. While the musketeers were marching back, and then advancing through their ranks, they were thus reloading, so a near constant weight of fire could be provided, particularly in defensive array. As complex as you found it, these guys had to do it while walking.

The Swedish method was to use only six ranks, but sometimes to fire the first three ranks all at once in a "salvee", where the front rank kneeled, the second leaned forward, and the third stood upright all discharging their weapons on command. Both systems had their advantages and disadvantages, the Dutch could keep up a more constant rate of fire, but required more movement throughout the reloading process, the Swedish probably was optimised for aggressive action - advance, discharge, swap to the next three ranks, and discharge again - providing two powerful and close-range volleys before a general advance, but this probably meant greater reload times overall, and certainly a drop in theoretical overall firepower compared to the Dutch method. However, it is quite possible that the convolution of nearly double the ranks in the Dutch system and the attendant complexity might have meant not much real difference in volley times without the salvee.

As to your basic question, the answer on an individual level is that no matchlock musketeer was every supposed to be alone and unprotected while reloading, he was invariably protected by pikemen from cavalry, and covered during his reloading time by fellow musketeers. The design of the matchlock, with its lit fuse, open firing pan and loose powder would also mean passing firearms between individuals would be risky at best - would you fancy being passed back and forth a heavy weapons with a lit fuse on it, which often came loose, while carrying a not insignificant amount of black powder in various containers about your person? When matchlock muskets operated without pike support, it was generally as 'commanded shot' and in specific circumstances, usually where enemy cavalry was negated - in support of allied cavalry against infantry when enemy cavalry had been driven off, or in defensive positions such as barricades or hedgerows, where the risk of facing an enemy cavalry charge was very low.

The problems of the matchlock musket were well recognised and during the period, fire-armed troops which needed a good rate of fire, but a more reliable item that did not need constant lit match as a firing source were the ones prioritised for wheellock and later 'firelock' (various types of flintlock) - dragoons muskets, cavalry carbines, elite firelock units, and the artillery guard (the latter guarded the powder supply and lit match was deemed...inadvisable).

I hope that answers your question, if you have any follow up ones just ask away.